you need to call a crisis center
Tell her mom secretly ! So she can help her by a doctor maybe
I feel really sorry for you. Its good she's getting help. You can't make someone stop doing things like that but you can spend time with her, comfort her and spend time talking about what makes her unhappy. I hope she gets better soon. Goodluck :-))
Maybe you could explain to your mum whats going on with your friend
Then I think your doing a great job :) I'll be really honest with asking... but Did she say she want help with this?
Buy her a gun it's quicker !
How is she hurting herself? Try to understand why instead of telling her she shouldnt.