BE THERE, BE THERE, BE THERE.
Self-harm is tricky, and the person who self harms could be very stubborn, and not want to accept anything you have to say (or at least appear like that.) I used to self harm, and all I wanted was for someone to tell me my feelings were legitimate, and help me find resources to feel better. I would say make sure the goal isn't "make her stop cutting" but the goal is "help her feel better about herself, and eventually she won't need to self harm anymore." All you gotta do is be a friend, and express your love and concern without making her feel guilty for doing it. Good luck to your friend, and I hope everything goes well with you guys.