About a year ago my best friend went ten or so days eating nothing but celery and grapes. It was... the scariest time for me. I've never been so worried about someone. After that though, we thought it was under control so we didn't really do anything about it. Unfortunately... It only went downhill from there. Months later we find out the anorexia never ended... And bulimia came into play when she WOULD eat. She started cutting herself, too, she was depressed. It just... was so awful. Worst experience of my life. It still hurts me to talk about.
From my experiences I just know... You HAVE to be paranoid with her, because she isn't always going to tell you the truth about what she's doing. You have to FORCE her to be healthy, not just push her to be healthy. You need to make her parents aware of what she's doing to herself. All things aside, her health is really the most important thing. This is really dangerous. It's not just not eating - it can go far beyond that. It's a mental illness, it's a sadness, it's a health issue... :| It's so much. And, waiting and trusting... It doesn't work
So encourage her to be healthy. Tell her it's harmful, and dangerous. Get her to understand. Then get her to go to her parents, and if she won't - then you go, because they need to know. It's a really touchy situation... But don't let it slide. Not that you would... But I did. And it was probably the worst thing I've ever done.
So. Good luck. If you ever need any more advice, feel free to PM me. :]