its not just what you eat that you gain weight. some people have a high metabolism, and that meaning no matter how much you eat it it cant you gain. but there is a way, first of all do not listen to the standard junk food diet because its the wrong type of way to gain "fat" and tell her its not "fat" your trying to gain its more muscle your trying to build, so tell her to try fiber drinks,food,bars ect. supplements and still eat in moderation. light walking, and try to not work out too much. sitting in front of a tv and relax, eat more calories not just in junk food but like meats, fruits, veggies, ect. hope this . my friend had the same problem. so this her.
answer is d hope this !