You should try to tell her that it's bad, that she coudl die from it. Say it ina worried, caring voice. Even if your friend gets mad; just tell him/her that you're there to talk to, that you want to help. If s/he says that they don't want your help, that they wouldn't understand, then just tell them "well I'm here to listen, if you need me." And just back off. Don't try to keep pressuring him/her to tell you. But anyway, I don't know what you really should do since I've never cut before, nor do I know anyone who cuts. But the best thing to do is to give them some space, but reassure them that you do care, that you would miss them alot if they died, that there are people who care about them and they should try to find a new hobby to keep them busy, instead of hurting themselves.
There is a member here who has started weightlifting (I believe) and stopped cutting. She knows who she is and I just want to tell her sorry for mentioning this. But I'm glad that she has found a new way to relieve stress and pain, in a healthier way.

congratulations to her.
MOVED TO RELATIONSHIPS