I have a friend who cuts/self harms?

He has been apparently doing this for a few months. He's been my bestfriend since kindergarden & we're in 9th grade now. I love him (as a friend) he's like the only person who understands me, & he makes me laugh. I care about him a lot. He's been cutting because his girlfriend is just a wench & is being horrible. She puts him down all the time. He already has low self confidence & she isn't helping. I want them to break up, but you know, I'm not gonna make them break up. I've stated my opinion on her, but he just loves her too much. He's cut everywhere, he even told me he tried to slit his throat. I'd talk to my mom about this, but our parents are bestfriends & he doesn't want his mom to know. Please help, how do I help him? :(