She could definitely have some sort of hormone balance, or it could just be that she doesn't get any exercise and her metabolism is very slow due to that. I've had a friend with the same issue, it was partially due to her being extremely anorexic for a year (and throwing her hormones off due to that) and partially due to having thyroid problems. She should definitely see a doctor to make sure it's not something like this that is causing weight gain.
As for exercise, I would suggest that she start off with cardio and then start incorporating weights into her routine. If she is not used to exercise, and doesn't have a hormone imbalance, her fat will should burning up pretty quickly. HIIT is amazing for fat loss, and I feel that this is the best cardio for fat loss. It is very intense.
When it comes to weight lifting, the best movements for her will be compound movements. Squats, Deadlifts, Clean and Press, Shoulder Press, Rows, Weight assisted Dips and Pull Ups (until she is strong enough to pull her own weight) Lunges, etc, etc. She should use a difficult weight and be pushing herself as hard as possible. For the first week of lifting and cardio she is going to be sooo sore, but as long as she is getting the right nutrients that initial pain should pass. Some supplements may help her out, such as protein powder, BCAAS and creatine.
When she starts training she will definitely be burning more calories, and therefore she will need to eat more and she should be capable of eating more food without gaining fat.
Diet is very important here as well. What does she normally eat every day? Unless she has a healthy diet she can't expect to see a whole lot of fat loss results.
Low glycemic fruit, lots of veggies, meat, eggs, cheese, chicken, beef, nuts, peanut butter, avocado and olive oil are all good choices.
