i'm sorry to hear about your situation; it's very hurtful to watch loved ones hurt themselves. you can't stop her from doing it, no matter how much you want to, so in that respect, there is nothing really you can do. it's her decision. (do not feel responsible for not being able to stop her, if that is something you're struggling with.)
do you know how she feels about hurting herself? is it something she wants to stop, but can't? in my experience, those hurting themselves, do not see any alternative in the moment of hurting, but feel bad about it afterwards and do not want to do it again. so, if this is the case, you can support her to find help. this behavior is very self destructive, it only seems to help in the particular moment, but in the long run it is damaging (on the outside the scars, on the inside the missing respect for yourself/your body) and she really should try to learn how to deal with unpleasant emotions differently. but again, she is the one, who needs to want to change.
if i were you i would try to just be open how i feel about it without blaming or judging her and offer her to talk about what makes her take actions like this. if she isn't ready to talk or change, the only thing you can do is accept it, try not to be battered by it and just be there for her.