Talk to her about it. Ask her what it means for her. Cutting means that something is wrong that she doesn't have the ability to cope with in a more constructive way, and she needs help from a professional. It needs to be taken seriously because it means that there is something wrong, but it is actually an effort to cope. And it probably isn't because of you. You can't take responsibility for someone else's choices, but you can be there for her and help her get help.

"Stop Walking On Eggshells" is one book that might be relevant. Remember, the cutting is not the main issue - it is an outward sign or symptom that she has a problem she can't deal with. She needs support in dealing with whatever the problem is.