I disapprove of my boyfriend best friend?

I don't think my boyfriend's best friend is a bad person, but he's leading my boyfriend in completely the wrong direction. He got my boyfriend into shoplifting and they smoke cannabis together. They actually got caught shoplifting and the police were involved. They also used to take codeine while at work for fun, and my boyfriend only stopped this after I told him how dangerous this was. His friend even passed out as they overdosed that badly. My boyfriend is trying to stop now but keeps getting very bad headaches which are only fixed by codeine so he still takes it. So I feel as though they just mess around at 'work.' A few weeks back my boyfriend was coming with me to the hospital. Then his friend asked him to go to Wales with him at the same time. My boyfriend still came with my but told his friend that he had an appointment because apparently me having an appointment wouldn't be an acceptable excuse not to go.

My boyfriend has also been round my house, then asked me to drive him home to go with his friend to pick up some cannabis. Then expected me to go and visit him as soon as he was home - this really annoyed me, especially as my fuel budget is very tight and he knows it.

I really feel as though my boyfriends friend is holding him back and as though he's not a true friend - but my boyfriend thinks he's like a brother, what sort of friend would act like that? I don't know what to do, if I give him the choice between him or me, I really don't know what'd he'd do.