Being a teen is tough anyway. Overeating can be a sign of anxiety. Have a facial together at home. Verbalize all her wonderful spiritual traits. Start in the pantry.
Teens are hungry all the time.
Keep low-fat snacks handy so that the emphasis is not starvation, but healthy choices. If you're stocked with low-fat goodies, nagging will be nil. Instead of movies and nintendo, have a family hike. Rent a tai-bo tape. Make your house a no-diet zone.