A lot of people will tell you report her or tell some adults, and that's not bad..but speaking from the perspective of someone who's been on the other end of that, it made me far worse because my choice was not considered. What I'm saying is this: if you want her to feel better, encourage her to seek help, and offer to help her with it, but don't force her. Because shoving someone in a psych ward against their will can hurt as often as it can help.

Actually bandages and peroxide is most of what you can do, but it might also be a good idea to clean the wound again in the morning and add a layer of anti-bacterial cream, and then wrap it again. (Assuming she doesn't need stitches and that's it's only a superficial cut).

Counseling might be a good place to start, but it needs to be her decision. i see so many people say "just report them, tell their (parents, SO, whatever), and they'll be put in a hospital psych ward and it'll fix everything"....but to someone in that place, choice is important. If she wants to seek help, it needs to be her decision....and it needs to be clear to her that you're not going to make her do anything....because despite the old "not keeping suicidal thoughts a secret" thing....the only person that ever helped me, the only one that ever made me less suicidal (even just temporarily).....was the only one that ever said, "I'm not going to have you throw in the back of a cop car and locked in a room alone and you lie and say you're "fixed" and normal like the rest of us."

If she doesn't want help or to share with anyone else just be there. Keep encouraging that idea of seeking some help, but let that be it for now.

Source: been suicidal virtually my entire teenage and adult years, had cops called on me several times and family threaten to have me institutionalized if I was ever suicidal, and friends threaten to have the cops foribly take me away to a hospital until I was "fixed" and "normal" again. Can confirm that all that did was make me withdraw and become even more suicidal.