How can I keep my best friend?

Cutting is serious business. I used to do it when I was younger and I had a friend in high school that did it also. It a sign of a SERIOUS problem and needs help. I know you said she isn't doing it yet, but just the fact that she is thinking about it is bad enough. Cutting is usually a sign of INTENSE emotional anguish that the person doesn't really know how to deal with so they cut themselves to turn the emotional pain into physical pain which they can deal with more easily. So your concerns are very real. My suggestion is to have a heart to heart with her and tell her how much you care about her and how worried you are for her. SHE NEEDS HELP.

I would tell her that you can't stand to see her in so much pain and you want her to get help. If she does that (which I warn you is pretty unlikely) then you probably don't need to tell anyone. But otherwise I think you need to just tell someone who will make sure to do something about it. I know you don't want to lose her friendship but if she kills herself then you will have no choice but to live without her now won't you? She very likely will get mad at you if you tell, but odds are once she is "well" again she will realize why you did it. I had another close friend in high school that had an eating disorder and when I busted her on it she was pretty ticked off at me. It hurt our friendship at first, but eventually we were back on track. Now here it is like 12 years later and we're still friends! I can't promise you that she'll forgive you but if you guys are really so close and she gets "well" she'll probably realize that this is one of the very few times when it is ok to betray a friend's trust.