This is very sad. These people are NOT taking it seriously. I am a teen so i know what you are dealing with. Try to make her feel better in every way possible. She has to know people are there for her. i could help you further if you tell me WHY she cuts??
I found it is usually for attention. Ignore it and tell her how stupid it is. If she is a real cutter then she would not have people viewing the wounds. People that aren't doing it for attention would never have it visable. It is usually an issue to a much bigger problem. Have to solve the main problem first.
There is always away don't give up on your friend. When she is not around try to hide the knife or what ever she is using to cut herself throw it away. Let her know that she doesn't need that kind of stuff. Those kind of things won't fix our problems. Let her know how much she means to you and that you will be there for her and be able to help her in the best way that you can. I wish for your friend the best.