I went through your question and my opinion would be that, weight gain has a lot of causes. if you feel her weight gain is not in proportion with the amount of food intake, it could be a problem. suggest you visit a doctor who would run a few tests and assess your problem. for example, thyroid problems can cause such problems in more in females than in males.
if you find all the tests normal, I suggest you don't resort to crash diet as it just increases the problem. therefore for a proper diet plan and workout visit a dietician. and also like to add that brisk walking helps along with dieting.
all the best.