You don't want to hurt her, so help. Maybe this idea could help. It's worth a try. If you want to stop cutting as well, then stop together. that is assuming that she wants to stop as well. Say in one week if you haven't cut then you do something fun.

It's an idea. You know what she is going through, you may not know how to help, but you can sure try.

I hope it works, but there's no fix all for everyone. You just have to keep trying, and never give up.