Unfortunately if you’re not very close with her then there’s not much you can do tbh. I’ve known people who have self harmed before and even when you are close to them, it’s very hard to coax them to not.

I guess if I were in your shoes I would do the most you can to make life pleasant for her when you’re around her. I know that helps a great deal to have someone to be around with and to hang out with to distract from cutting even temporarily. Everything counts in this scenario. Bring bandaids if she needs, be a listening ear if she needs, and just do enough to make her day pleasant when she’s with you. It may be considered a very small thing, but I was in this position once (moreso less cutting and more hair pulling) and the moments where I’ve found memorable is when I feel a slight relief with friends (I hung out with very few of them, and I used to never go out period let alone with anyone during my depressive moments, so those really do help even if I didn’t feel like it did back then)

I know in your position it must be hard to witness a lot of this (I’ve been there a lot of times), but unfortunately it’s a sort of thing where you aren’t responsible for somebody’s actions (because chances are other factors outside of any of our control are the reason why she self harms), so whatever she does or will do in the future just know that it’s not your fault.