As someone who myself was disapproved of by my wife's friends ( some of them, not all by any means) I'd be as civil as you can be to him to be honest. I was called "manipulative", "psychotic", "dangerous", "evil" and a host of other inaccurate and choice descriptions by a gaggle of people who didn't know me. Eventually some of her friends became incompatible with our relationship full stop and she, ( not I), made the choice to exclude them further from her life. It was not something I particularly wanted her to do as predictably they then went running round all over the place saying I'd either bullied her into it or she was "blinded by love", niether of which was true. She just made an informed choice and found particularly the latter statement to be really rude and condescending, so if you want to hang onto your friend I'd make as much effort as possible with her nasty bf, as you may be wrong about him but even if you are right, it's her life, her decision and her mistake to make. Just be around afterwards if she needs you to be, and as JustSia said don't say "I told you so" :)