dontwaitforever's reply history at Help.com

dont take away her razors, there are a million other things she could use or she could resort to stealing more, my advice, just tell her you care and you’re there for her if she ever wants to talk, day or night, try to get her to make a promise to you to call you or at least try to when she wants to cut that way you can distract her or talk to her about why she wants to, maybe get to the root of the problem after a while
dont tell anyone, she may act like she doesnt care but my guess is that she does, she cares a lot and telling people could just make her more stressed making the urge to cut stronger
i hope this helped :)