As a freshman college student I can help you out here. Teenagers: we're stubborn. I learned on my first 4 day weekend when I went home for the first time that 10 pounds found their way to my tummy. It's difficult all of sudden having to choose EVERY single thing that goes into your stomach. My parents shopped for the family, so I never really had to go to the grocery store alone, or with just a friend and choose what was "best" or what tasted the best. Maybe giving her money, or making her use her own and dropping her off at the local grocery store and let her choose what she wants to eat, see how much food really costs, and then see how she feels at the end of the week. It might seen a bit expensive, but in the long run, she'll be a smart shopper, and a healthier eater.
"Go as yourself and never as anyone else" - A wise, old, friend.
"If you define yourself by what you're not made of, how do you know what are?" -Me