Maybe I'm missing something. She said 'help.' She didn't ask to be alone with him. Is your daughter (an adult) not comfortable with having your wife there while she watches your son,
My personal opinion on asking the 18 year old to supervise her own mother with visits is not fair to that 18 year old. That 18 year old should never be asked to be placed in the middle of something like that. It's best to keep the 18 year old totally out of that situation.
I also agree with the telling an addict no and receiving a not fully in recovery response, but you handled it extremely well.
I also think letting her follow through with her decision to take a bus instead of you driving her without any attempt to try and change her mind was wonderful.