[Jack_]

Yeah I had a couple of people confide in me about stuff like this when I was your age, it's really not nice :/

1) You need to try your absolute best to convince her that what she is doing will not help anything and there is no need for her to be doing it, insist that you will be there for her to talk about any problems she has and listen to her/offer advice on whatever she might tell you about

2) Try and get her to tell an adult she knows, either her parents, siblings, teachers, maybe even a school councillor. You need to make sure she does this, if not you're going to have to do it on her behalf. Better to stop this early before it goes any further

Good luck