As a previous self-harmer, my friends made attempts to make me stop and some just plain insulted me about it, saying it was gross, or disturbing like you think so. I don't advise being mean about it, because that honestly just makes it worse. Often people can't control how they cope with things and sometimes they might know themselves that it's bad but cannot stop themselves. I think you should just stand by them and try to be a good friend. Help them cope, too, besides self harming. Let them know you're there for them and that there are other means of coping. Tell them how you feel, that you're upset that they do such a thing and you're concerned. But make sure you're careful with your words. I hope things work out regardless of what you choose to do.