What do you tell a friend when she is cutting herself again?

Okay so my friend and her boyfriend just broke up because she got jeolous of her boyfriend hanging out with another girl at the Skate Center when she invited her boyfriend to go anyways but he barely even talked to her the whole time they were there. Now he broke up with her because she got jeolous he also called the other girl hotter then his girlfriend and that pisses me off because she really isnt and now she is cutting herself again and I'm the only person she has told and now I don't know how to make her stop. How do I make her stop I'm going to her house, but I don't kow what to tell her besides "Stop!" So please if you have had issues like this or have have had similar issues please do help :(