I understand your concern for your wife. If this is recent, you need to visit a qualified physician at the earliest and get the recommended tests done to rule out any emerging possibilities.
However, if not then you must understand that some people are built in a certain way owing to family and genetic history. Diet management has been proven to help maintain a healthy lifestyle. Eating calorie dense foods thinking that it would lead to weight gain is not correct. You must:-
a) Set your metabolism levels right by following a strict dietary regime and setting your body clock right
b) Include foods that are nutrient as well as calorie dense
Next Steps
Have 6-8 small but frequent meals. Include things like almond and banana shake in your diet, add dates to it. Add roasted sesame seeds to your vegetables. For more action points, you can contact me on direct consult or visit me to get a customised diet plan suiting your needs.