I don't feel comfortable with him doing this...

So my fiancé works as a night shift nurse at a nursing home for elderly. There is an older maintenance guy there who is kind of a fixer upper for the local people, an auto mechanic, and he also sells weed on the side. My fiancés friend buys from him, and since my fiancé works with this guy, he distributes the weed from this guy to his friend. 
I understand that weed can be safe if used responsibly but in a WORK environment?? I just feel  really uncomfortable with him doing this, but I can't tell him what he can and cannot do, my fiancé smokes occasionally and knows I don't like it, but I don't want to act like his mother either. Should I just let it go?