How would you go about telling a good friend that the girl he is with is bad for him without losing that frien?

You tell him what you know, give examples to back up what you're saying, and speak in a calm tone of voice. Be prepared for him not to listen. I'm going through the same exact situation right now (my best friend, a girl, wants to go back out with a guy who hit her), and as much as people tell her that he's terrible for her and no one likes him, she wants to be with him so bad.