My bestfriend just told me that she has cut.

There's this girl in her year ( she's the year below me) that bullies her in school . She calls her names and is turning her friends against her . Recently she told her other friend a secret that she was bi sexual and that ended up getting out . On the school bus there's a girl in my year who always comments on how my bestfriend looks , calls her annoying , and always says rude snarky comments to her . It kills me every time I hear this because I can't say anything since she's one of the 'queen bees' . Just today my friend sent me a picture saying that she had cut with an i image of it . She asked me not to tell anyone about it . I asked if she wanted to do it again and she replied saying kind off . I feel powerless and confused about what I can do to help her . I really don't want to betray her and tell someone because that will probably make her want to do it more since I went behind her back . I'm really scared and confused . What should I do !?