I'd address the person about the cutting. Not in a "You're so dumb/don't do that" kind of way. Just sort of bring it up "Hey, what's up with your arm?" and go from there. If they're like me and make up an excuse and brush it off as if there's nothing wrong, leave it like that. Without knowing or understanding why, it's hard to help, but what I'd try to do is pay extra attention to them. Make them feel special, make sure they know they are important to you. That they matter, that you enjoy there presence. If you know the person is cutting they're not being overly secretive about, and is aware people may find out (I don't know, maybe they outwardly told you about it.) It's not necessary a cry for help, but they are not against people knowing.
I wouldn't go to get your friend help unless it starts to become dangerous, like you see deep cuts down the arm instead of across. Cutting isn't safe, but your friend would benefit more from you caring and paying attention to them than if they are shipped off to some mental hospital. The little things really matter, so even if you don't think something is helping, it is.