I used to be a cutter. Started at 12 and stopped at 19. The only thing that is going to help this girl is therapy. There is something going on with her emotionally that is making her want to do this. As you can read from some of the responses this is a very common thing- it is just kept hidden.
Point blank nothing is going to help her unless she wants to help herself. She wont stop because her Mom wants her to, or even if she promises she won't do it again. She will. Until she is "fixed" so to speak.
When my Mom figured out I was cutting, I told her I had stopped. But really I had just moved to different body parts. My ankles, hips, stomach. Places that could be hidden more easily.
She (daughter and friend) will need your support through this because it is going to be a long long process.