I cut and my now ex boyfriend cut too. He knew about me before I knew about him. And I scared him constantly. He was always afraid of how bad it was. He stopped long before we started dating so he was here for me. But you obviously don't have that knowledge.

The only advice I have it to be supportive. Don't tell her that its wrong. She knows. Just kiss her wrists and tell her you are here for her. And that she doesn't need to feel that way because she has you... Tell her that...

Just make sure she is safe an isn't goin too deep. She needs your love and support

Good luck