Look at www.myfitnesspal.com She can type in everything she is eating at the moment. Without realising, she will probably be eating way less than she needs, and exercising will burn off more too. There are some foods which are high in calories but healthy, such as salmon and avocado. She can add "Good Oil" to salads and vegetables. She can up her quantities and have healthy snacks throughout the day. I think she should continue at the gym as it will help her with the stress, but talk to a physio or personal trainer about doing strength building exercises rather than sweaty fat burning which will make her lose more weight. MyFitnesspal will show her which foods she can eat which will help her gain weight in a healthy way. Any decent GP should refer her to a nutritionalist who can give her this info. Also she is 20. 20 is young. Very few people stay their 20 year-old weight, even if they stay slim. She will fill out, get bigger hips and waist etc over the next 5 years. The other thing is to not nag her about it. She will only get more paranoid about her body. Focus instead on doing happy things and having fun together.