One word: Exercise. (Strictly speaking, your best option is diverting that unspent energy to something more useful than fat gain.)
Ideally not some silly cardio regimen; aim for resistance training, preferably with free weights. It's good for the body, and some modicum of muscle mass looks great on everyone. I'd recommend Stronglifts 5x5 if she can manage the empty barbell, perhaps some dumbbell program if she's a tiny ana.
Do keep track of calorie intake. 'Normal eating' doesn't mean anything, and people tend to focus a lot more on metabolism than warranted..