It's a pretty simple question, but I can't find a definite answer:
Can the warrior try to save against the harmful spell?
According to a very strict interpretation (my interpretation, in this case, obviously) of the RAW, sure, the warrior can choose to save against the Wound spell. Here's why:
Voluntarily Giving up a Saving Throw: A creature can voluntarily forego a saving throw and willingly accept a spell’s result.
– Pathfinder, Saving Throw
Emphasis mine. The result of the spell is that "you lose X hit points", and the question is: "do you willingly accept that?"
Of course, it could be argued that by "result" the RAW refers to the "result as expected by the character (not the player)"... but that's already adding to the text and interpreting it.
Casting goes like this, in my understanding: Caster tries to, and if nothing interferes, casts the spell, spell takes effect, that is, its (intended) result becomes known, saves and whatnot are applied to said result, result is applied.
Even Will saves don't necessarily reflect conscious decisions, choices. They are pretty instantaneous - and the nanosecond the character realizes the incoming energy is cold instead of warm (or something along these lines), her Will, her self defense comes into play - unless her Player opts for giving it up and taking the damage because said Player feels the character trusted the spell's source truly, madly deeply. :)
That's my take. The warrior gets to save.
It is a conscious choice to willingly fail a spell against a Harmless spell; you are entitled to attempt a save if you like. In fact, some effects require you to do so as a form of penalty (you might save against the healing/buff you need).
Anyway, there’s an opportunity for a Spellcraft check to recognize a spell; an untrained warrior, however, could not recognize the difference. The choice to willingly fail a save takes place before any spell effect does, meaning the warrior would not realize his mistake until it was too late.
That is, assuming you’re playing by RAW. The rules don’t mention any way besides Spellcraft for recognizing a spell, but most settings flavor positive energy (which a cure spell channels) and negative energy (which an inflict spell channels) as being bright white or black/red/purple/evil, respectively; one would expect the warrior could see that and react to it. For that matter, even if the channeling doesn’t involve visible energy (or maybe the energy is only seen after it’s too late, whatever), if he’s been traveling with the cleric for a long time, you’d expect he’d recognize the different chant/gesture and that might at the very least give him reason to be suspicious, i.e. get another Sense Motive check at a bonus. Though something like “it’s just a more powerful version” would probably quiet that concern, but again, Bluff check vs. Sense Motive etc.
Think back to the last time (or any time) you remember someone casting a healing spell. Did you say that you're voluntarily failing your save?
By RAW, that's what is happening. It's harmless, so you can choose to fail it. Everyone just assumes that you are automatically, because making people actually say that every time sucks. What happens if you forget to? Are you now obligated to make the save against a healing spell? That adds terribly tedious nonsense to the game, and so nobody does it.
It's just assumed by pretty much every player that if the spell is harmless you're voluntarily failing the save, and if it's harmful, you're not. If you allow something like this instead, you're going to break that. The net result is that people will have to start declaring what they're doing, because otherwise someone else will try this trick and an arugment will break out over if they do or don't get a save. (Or someone will heal, and someone else will say "oh you didn't declare it, no healing for you!")
God help you if one PC decides to use it to kill another PC by offering to heal them and then casting a save or die. That's the kind of stuff that starts fights at the table.
So just don't allow it, no matter if a strict reading of RAW says that it might work. It's going to make the rest of the game worse.
Based on that, the answer to the question should be: Yes, let the warrior roll the save.
The Rules for Aiming Spells state:
Some spells restrict you to willing targets only. Declaring yourself as a willing target is something that can be done at any time (even if you’re flat-footed or it isn’t your turn). Unconscious creatures are automatically considered willing, but a character who is conscious but immobile or helpless (such as one who is bound, cowering, grappling, paralyzed, pinned, or stunned) is not automatically willing.
If we then look at Saving Throws we see:
Voluntarily Giving up a Saving Throw
A creature can voluntarily forgo a saving throw and willingly accept a spell’s result. Even a character with a special resistance to magic can suppress this quality.
Therefore, if the target of the spell is willing, they are automatically affected by the spell (pending other special abilities such as spell resistance).
The spell may have (harmless) in the saving throw line, this means:
The spell is usually beneficial, not harmful, but a targeted creature can attempt a saving throw if it desires.
However, this information is not known to the target, instead they would have to succeed at a spellcraft check to identify the spell as it's being cast or else be unaware of this or any other information about the spell.
When you cast a spell, it is noticeable that one is being cast, and the targets (or target area) are chosen for it. At this point if the spell offers a saving throw to resist its effects, the targets (or any creatures in it's area of effect) of the spell have the option of choosing to voluntarily give up their saving throw, or make a saving throw against the effects of the spell, they are allowed to make a spellcraft check to identify the spell before choosing.
The rules are actually very clear that you cannot easily trick people into forgoing a saving throw without evil GM plot device powers that can be used maybe once.
If the spell actually being cast is (harmless), the target is assumed to forgo the save unless the player specifically declares intent to save. Therefore the player never needs to declare that s/he is dropping the save if a harmless spell is cast. This protects the player. If the spell being cast is not harmless, the player must declare that they are dropping the save, unless the plot requires it to fail, and the GM should think long and hard about doing that.
Even non intelligent monsters benefit from this same protection (in fact they are too dumb to fool).
Personally, unless I was running an evil game, I'd inform the player that such an action, were it actually intended to harm (as in the OP's example) is an alignment violation. Yes this informs the player that s/he needs to save. This is not a problem because the player gets the save anyway, by the rules. If the player was pranking the other player I wouldn't give the alignment warning if the player was chaotic alignment, but I'd still tell the victim to save. If the player insists that they are attempting to get the other character to drop the save, see below.
The only reason anyone should be dicked out of a saving throw is if the plot requires it (e.g. the entire low level party is incapacitated with a heightened sleep spell, which they are most likely to fail the save on anyway, to set up the adventure). Once the plot is kicked off, it is unfair for players to be denied the protection of the saving throw for any reason. The rule book I read this in said "Only in cases of extreme trickery should the target be denied a saving throw." Extreme trickery without GM fiat means beat the opposing sense motive by 10 or more from a mechanical standpoint, in my opinion. If they win, but by less then ten, the victim doesn't see through the lie, but still gets to roll their save. And if the player ever gets denied a saving throw due to trickery, or sees it happen, they gain a +10 circumstance bonus on sense motives versus bluffs to get a person to drop the saving throw. Fool me once….
I always consider the (harmless) and (object) tags to be part of the spell itself. For example, undead are immune to any effect that requires a Fortitude save unless it has either of those tags. This means that the tags are inherent to the spell.
So since you can choose to forgo a save we already know that a character knows that a spell is cast on them. They probably also know what kind of save they need to make, do they dodge, brace themselves or resist the force on their mind. It isn't that much of a leap to say that along with the type of the save they need to make they also know whether or not it is harmless.
So the story of the Cleric and the Warrior in the example would give the Warrior the information that a spell is cast on him by the Cleric (it is a touch spell so he knows it's the Cleric) that requires a Will save and that is not harmless. If he makes the save he still gets half damage because of the failed Sense Motive. If he had succeeded at the Sense Motive the Cleric would have also needed to make an attack roll. If he also succeeded at an Initiative check he could actually have readied an attack to disrupt the spell entirely, followed by an attack of opportunity to try and disrupt it again.
