I've been a dedicated cutter for the last 13 years and all I can tell you is to just listen to your friend, be there for them, and don't take it personally.

When people want to understand my cutting I ask them if they understand alcoholism. For me there isn't much difference. What I mean is it's an addiction for me, the only difference is the form it comes in.

Let your friend know you are always there for them if they need to talk. You can't do anything for them if they don't want to stop. Your support may influence their behavior. There's a chance. Also, try telling them how you feel about their cutting. Remember to support them at the same time.

I hope this helps some.