My friend started cutting over a boy...
Today I was on webcam with her she was crying in pain from cutting her legs . I don't know what to do . I want to tell my mam but I don't want to lose her as a friend , but id rather her hate me then to lose her completely , what should I do ? She said shed throw herself out a window if I told , but I can't sleep or eat with worry for my best friend and the worst part is she's 12 .

She needs help. Her parents need to know. They need to stay on top of this as suicide threats need to be taken for real. Also with them being 12 any scars created now will grow with time. This needs to be stopped right away.
Do the right thing.

I think this is something well beyond your control. Cutting and suicide attempts are a very serious matter.

You need to inform her parents and let them take the right steps for her, instead of this weight being on your shoulders.

I understand why you are sick with worry over this, I would be too.

What you have to realize is, this is just not her venting, or being upset. You have seen what she has done, and because you know what she has done, it is a fact. You are not guessing, or imagining anything.

Had she wished to keep it a secret, she would not have told you.

As a friend, my opinion is, you have an obligation simply as a human being, to know that someone is desperately in need of help.

Tell your mother immediately, and enlist her help to go, in person, to visit your friends mother.

If you do not say anything, this will not be resolved, and she will not get the help and support she needs.

All you need to do here, is the right thing.