I would think the only way diets would cause obesity is if they are unhealthy diets that end up altering the body's metabolism rate. For example, severely restricting calories below about 1200 a day (depending on activity level) can put your body into "starvation" mode so that it will hold on to every single morsel of food put into it. Then once a person reaches their goal and goes back to eating normally, they just put on all the weight over again.
A proper diet, one that can be safely maintained long term (because in most cases losing weight through diet changes SHOULD be a lifestyle change towards eating healthily), will not cause these effects.
What kind of diet is your wife on? Its possible you have cause to worry, but also possible she's using a good diet and is just trying to improve herself.