Try to encourage them to go to a professional about it, and talk to one yourself to see what they suggest. Ask them to stop but don't do anything daft like tell them to or threaten anything, I don't think you would but it doesn't hurt to point it out. Don't drop them in it with anyone though, like telling their parents or teacher -> Usually inflames the situation. If your school has a counsellor or a nurse talk to them in confidentiality and they should give you some good advice, however if it seems like they are going to do something even worse do get help asap.