Thanks for your query,
Irregular menstrual cycles and obesity may be a sign of PCOS
. She will have to undergo some blood test
scan for its confirmation. However in case of PCOS , the main treatment modality remains control of body weight. So its good that you are concern with weight loss
For control of weight, avoid ghee, butter, junk foods, oils , high calorie diet etc. She will have to perform some sort of aerobic excercise for atleast for half hour daily. In case of excess obesity some medicines may be taken in supervision of doctors, better avoided.
Hope it helps..