My first thought, before the party was cancelled, was whatever he wants to do he should do. Why? Because if he's going to relapse it would be easier for you to know it right away. Save you from months of tension.
I'm not in your shoes though, and I don't want to sound unsympathetic. I used to hate it when my Dad brought home beer because my parents would drink all night and it wasn't pretty. But I was a kid then and couldn't get away.