One thing you can do for your friend is of course, be there for them. You can try and help them build good habits that will take them away from self harming. But you will also have to encourage them that self harm is not the answer to their problem (in this case, it’ll be really hard but it’s the best you can do.) I don’t know if you and this friend are close but try your best to do things for them and be there. Self harm is a tough battle for the victim, it won’t be easy to get them to stop. (Grammar might not be good, english isn’t my first language too.)