I think you should talk to her. From the sounds of it, her friends seem like people who she might not talk to about this stuff. Just when you're together, let her know that you noticed the cuts and that you just want her to know she's not alone, and she ever wants to talk about it, you're around. That gives her an outlet available if she wants it. I just know that the benefits for both of you far outweigh the possibilities from doing nothing.