I dated a girl that cut herself. I found by grabbing her wrist one time and she pulled away in a bit of pain. I remember being upset and trying to get her attention and when I found out she cut herself, I literally let go of my problems on the spot and talked to her about her problems. Just because they cut themselves, doesn't mean they're not dating material. I learned that a lot of people who cut themselves usually are ignored or don't have anyone to talk to about thei problems. I would still be with her if our personalities clicked but unfortunately, we did not click very well.
I'd want to help her find a way to deal with her issues without hurting herself like that. I'd try to understand and want to be there to support her but it isn't something I'd just support blindly to ignore.