a) its gonna happen
b) its gonna be hard/painful and
c) there's nothing we can do about it
From my own life, there is this.
I once came to the point in my life where I realized that drugs and drink were a part of all my friend/relation-ships. That was hard to do, because first I had to realize that I had to stop all drugs and drink for a time.
Since these were a part of ALL my relationships in those days, it meant one of two things:
these relationships will a) change, or b) end. Even though these people defined me to an extent; they validated me. The history, loyalty, and commonalities were deep and long.
Brothers and sisters in arms, the friends I made in school, who have known me and seen me at my finest and our worst. These people were also my conscience and my mirror, and I was theirs as we grew and found our way.
I don't think it's just a guy thing, or a Survivor thing either. Ok, maybe SA makes it worse, if these are the only males that we had friendship and intimacy with.
So back to your list. I made a promise. Only to myself because I had no significant other at that time (one of the reasons I needed to get and stay sober).
I promised myself that I would see these friends, tell them upfront and clearly, I can't (smoke, drink) anymore. I would ask that they not offer, or that they "don't let me". If I ended up doing those things, then that was the last time I'd hang out with them, until I could be firm in refusing. Case closed.
I didn't blame myself or them if I got high. I just decided that I was weak with them, and they weren't strong enough for me just then.
So maybe you could ask your BF to make a similar promise to himself and to you. Remind him that with the legal and relationship pressures and changes going on that it will be very tempting to escape into old behaviors. Outgrown, unnecessary behaviors.
I think that with your help and love he is very close to this promise. If the friend in question really cares about HIS friend (your BF) even a just a little, then he will understand and support your BF in this.
Good luck, and congratulations on the engagement and the house hunting.