Well the only thing you can do is talk to them about it. Tell them how you feel and try to find out why they started it in the first place. Other then that nothing much really, they have to stop themselves. Their in charge of their own actions. You cant stop someone from doing something, they have to stop themselves.

honestly i have a lot of experience in this area. Theres nothing you can do. You cant force them to get help or stop. You can talk to them and tell them your concerns but other than that theres really nothing. If they live with their parents tell their parents and family try and hold an intervention if this person is bad enough. Dont walk Go to a meeting with them. If they wont go go to a meeting and ask people there to help you. Tell them you need some of them to 12 step your friend

its true. people who are addicted (it doesnt matter what to... it can be anything) wont stop unless they make the choice themselves to. a lot of times people just dont want to. help them understand your concern and remind them the affects it can have on family and friends. try your best to keep them heading in the right direction but dont be too pushy about it. when you take away the object of someones addiction or push them too hard about it they will almost always get angry. and being that its your friend im sure you dont want that. if your going to tell their family be prepared for the consequences. being a person thats addicted to cutting (which in a lot of ways is like drugs) myself i know how infuriating it is to have a friend go behind your back to tell your family. warn them that if they dont stop or tell them them self you are going to have to. give them a few warnings. addiction is a complicated scary thing but it is possible to work through it.