*IMPORTANT* What should I do about my friend?

The other day I was at a pool party with my friends and my one friends (not naming) confided in us and told us that last year she had some problems and did something's she regrets. She said that she was uncomfortable with her weight and decided she was going to starve herself, and she threw up anything she did eat. Also she told us that she cut herself because she was so upset with herself. She says now that she completely regrets it and doesn't think shes gonna have a relapse. The thing that worries me is that every since she stopped last autumn she gained the weight back. (she hasn't tryed to lose weight in other ways) I know it's better that she stopped but I'm afraid that she will notice the weight gain and do it all again. She never told anyone but us that day. Not even her parents. She said she's not ready to tell her parents. I want to help her cause she's one of my bestfriends, but I don't want to bring up the subject to her or my parents or her parents. because If I tell her parents they probably wouldn't do anything because her parents made her very independent and trust her to do her own things. And If I tell my parents they might not want me to hangout with her anymore and she obviously needs me now more than ever. And its a sore subject to her and I can't discuss it with her. Schools out and we can't tell the guidance councelers so that won't help. I want to help because it might get worse and she might do things worse than what she did last year. I want to help her imediatley. As soon as I find a way to help her efficiently I will. ASAP. What should I do? Please help?