Just about six months ago I was in your exact same situation, I was lucky enough to get her to stop a few months after she started, and I would recommend the same to you, show her openly how much it hurts you, and if others know, remind her that it hurts them just as much before she does it, explain to her that it doesn't only physically harm one, but self harm does have psychological effects, it can hurt one psychologically, you don't really have to study psychology to know this, anyone who's self harmed for a pro-longed period of time knows this. Do anything that won't hurt her or get her in trouble to get her to stop, make a deal with her (this is how I got my friend to stop) that if she stops, you will too, and if one of you breaks, both of you do, it gives a reason to keep yourself from relapsing.