Try checking out other sources on the SI section on this site, it might provide you with helpful suggestions because it can be hard to deal with people who SI. Whether you want to give them positive affirmation for not cutting, or whether you want to talk to them about it.

I would suggest leaving yourself open for them to come talk to you if they need someone to talk to. Or just let them know, there are websites that can suggest to you what you should say to them or ask them in order to help them rather than make them feel like you don't understand or something like that. Ratting them out won't necessarily help them, but perhaps you could talk to a counselor, saying that you have friends that do this and you want to know what to do because you're concerned about them.