What should I do about my neighbor offering my recovering addict husband drugs?

My husband is a recovering alcoholic and drug addict. He has slipped up and smoked pot in these past few months (I know a lot of people like to smoke dope, but an addict isn't supposed to use ANYTHING, so please keep following here okay) and we actually almost split up for a while over it. Anyways, we live in an apartment and our upstairs neighbors have befriended us. Before the guy and his finace got together, she told him that she wanted a sober man. He lied and told her that he was one and now they fight 24/7 because she keeps catching him with pills, pot, and baggies of random things. He comes down by my dh and offers to smoke him up, and dh has accepted a few times now. Today I just found out that the neighbor bought some coke and offered it to my husband, who thankfully turned it down (trust me, I'd know if he did coke). Now I'm kind-of pissed. My dh is friends with this guy and we have to see him every day, but he's an enabler. Should I tell his clueless fiance? My husband was doing so good and now I think I may have to leave him until he can get himself back on track. What would you do in my situation?