After losing 65 pounds myself and helping my wife lose 40 pounds, I went into fitness training and focused specifically on coaching people with a lot of weight to lose. I say this not to brag but so you know the position I'm saying this from, one not just of my personal experience but also the experience of working with others.
First, plateaus happen. They just do. The body is extremely complex and not predictable and sometimes you may go several weeks without losing weight. Sometimes you may even gain it. It's not always fat, but in fact can be water weight and other factors that are sometimes related to the stress of dropping the weight.
I'm not sure what you are using to gauge progress but weighing daily is fine to see the trend over time but making decisions should only happen weekly or bi-weekly as that's enough time to see a real change. So, I wouldn't call a plateau a "real plateau" unless it's lasted at least 2 - 3 weeks. This is where many people get frustrated and give up, so the key here is "don't give in." Keep pushing.
If you get past that point, obviously something has to change. You really have two main opportunities to do this: training and nutrition.
With nutrition, sometimes it may seem counter-intuitive but you may want to increase calories slightly to give your body a break from extreme dieting. While I know Weight Watchers is based on points I'm not a fan because I don't think it promotes the right types and combinations of foods. If anything, one smart move will be to add as many fresh or lightly steamed vegetables as possible. I know so many people who don't believe how powerful it is to add tons of vegetables and are amazed when they do it and break through a plateau. When I dropped my weight, I would literally bring a whole bag of frozen broccoli or green beans into the office and cook it and eat it throughout the day.
The other is fiber. It's easy to eat "x" calories with processed foods, bars, pre-made meals, but one thing you'll find is a lot of those lack fiber. To the extent you can switch to as natural as possible with your foods - pick whole oats over processed cereal, pick old fashioned oatmeal over instant, pick fresh fruit over fruit juice, etc - you'll also increase your fiber intake.
Last but not least, this one is very specific to women. Unfortunately, history created a huge misperception of fats as evil and people have shied away. People are finally starting to learn fats are not created equal and that there are healthy fats. They are still shy of these fats however and too many people work on reduced fat as opposed to switching to quality fat. Women require a steady intake of healthy fats and oils as their estrogen production is tied to it. I can't tell you how many women I coached would come to me AMAZED after just a few weeks of adding fats because despite the extra calories, they suddenly started dropping weight, lost cravings, even saw reduced PMS symptoms and more regular menstrual cycles. If your wife is not eating foods with healthy fats - salmon, other cold water fish, even getting 2% as opposed to 0 fat dairy - I'd suggest she try that out and even consider supplementation with fish oil or flax oil. That can be a major plateau buster for someone not getting a quality intake of healthy fats.
For training, changing the style of training is key. Most people do slow, steady state (long jogs, bikes, etc) and switching to interval-style training can be a booster (you can Bing "HIIT" and find out more). Hopefully her trainer isn't sticking to the same resistance routine but is keeping that fresh as well. If she's not into cardio, perhaps a cardio tape or something that has her moving in a different way will help.
The key is change - change the nutrition, eat healthy options but don't eat the exact same meal every day, and change the training. If she needs to add a little more and her schedule permits, do it! Keep pushing through and you can and will bust that plateau.
Best of health and success to you.
Jeremy
