Almost two weeks ago, my best friend told me she made herself throw up. A lot. Apparently she'd been doing it for about two months when she told me. She hardly eats anything. Just enough so people won't call her anorexic, then she makes herself throw up. I tried to help at first, but I didn't really know how. We've really only been friends for like six months, and it's been pretty patchy. I don't know what to do.

A while back, probably a little after she started, she and a friend told me (in an instant message on the internet) that they made themselves throw up. I got really, really mad (a mistake, I know) and then they told me they were just joking. Apparently, the girl I'm closer to wasn't joking. But she didn't want me to be that mad at her all the time.
Well, to get back at them, and to see if they cared about me, I told them I'd made myself throw up. But I said only once. It was lie, and I regret it.
For like two weeks after that, my friend was mad at me. Constantly. Calling me all manner of names. Liar, hypocrite, health forum, on and on. That whole time she was mad at me for 'making myself throw up' she made herself throw up every night. Yes, I was a liar, but she was more of a hypocrite and a health forum.

She told me that she thought I made myself throw up more often, and she figured, "If it's okay for her to do it, it's okay for me."

Well, now, she's mad at me again. We've been emailing (none of our other friends know). She says I'm treating her like a baby, or a mental patient. She told me to stop caring, so I did. I still care, more than ever, but I just kind of ignored her at school. Not to the point that anyone would notice, but I just wasn't going out of my way to talk to her, like I usually do. When we did talk, it was just surface level.
Now she's calling me shallow, saying I never cared, and, in the most recent email, she said, "just wait till my stomach gets ulcers and explodes. then youll wish you'd done it differantly"

As far as she knows, only four people know: me, her mom, her dad, and our teacher (who is really cool and can help her if she'll talk to her). I told my counselor, but only because the whole deal was really weighing down on me.
When she told me, I wasn't mad at all. I just sat there, listening. Like I almost always do for everybody. She asked our teacher to tell her parents, and her mom cried and locked herself in her room, and her dad gave her a hug and said 'I love you.'

Anyways, now she's mad 'cause I don't listen or care or something like that. I'm trying really hard not to scream "YOU NEVER LISTEN TO ME, WHY SHOULD I LISTEN TO YOU??"

What do I do? She won't even talk to me except on email now, and I feel like she's not even reading my emails before she replies.
Help me! I don't know what to do.