In this scene (link) from The Big Bang Theory Sheldon corrects Penny saying she forgot to use the subjunctive:
Penny: Leonard, it's three o'clock in the morning! I don't care if Richard Feynman was a purple leprechaun who lived in my butt!
Sheldon: Penny meant "If he were a purple leprechaun. Penny forgot to use the subjunctive.
However, in the comments people argue about whether he was actually right or wrong to do that. I've tried to research the topic but all I found was more discussions and disagreement on the matter on other websites.
So which one is it? Should one use the subjunctive in this case or was what Penny said correct? Is it possible that both options would be grammatically correct but convey a different meaning?