The therapists are absolutely correct. That is one of the major issues with rehab, people go back to the same people/places/routines/etc and the temptation becomes too great.
Add to the recovery the emotional nature of your bf's grandmother passing away, this is a really tough time for him. Would it be possible for you to go spend some time with him? Help him out with his grandfather, be there with him when he is around his friend, etc? I think that would do wonders for your relationship.
It would also give you a chance to talk to him in person. Let him know that his abuse in the past bothered you, but you are really proud that he is sober. However, point out the ACTIVITIES that are concerning you about possible relapse. Try to stay away from criticizing his friend, because I think that would just make him defensive. But make sure he understand that you care about him and don't want anything to happen to him. Hopefully, this type of open and honest communication with him will make him see the light a bit and ease off on his concerning behavior with his buddy.
Good luck.
Brought to you by Dating With Devon!