My friend just told me that she cuts what do I do? I'd like to support her in the best way possible.

Try to have her understand that you’re not judging her, you want to help her get through recovery from self harm. You might want to tell an adult if you feel that she needs it. Self harm is not something to joke about, especially cutting because if she accidentally hits a vein or artery she could do some major damage. If not taking her life. So what I’m saying is be there for her and try to get her the help that she needs. If that’s telling an adult or having her go to therapy or just being there when she feels the need to cut.