Finn joined Robert, who was already drinking. And Robert offered to buy Finn a bottle of vintage seltzer. If Finn is so fragile he can't be around drinkers, he shouldn't be in a gastropub in the first place.
I wonder if they're starting to transition him to more of a shoulder to cry on, the town bartender role. It's probably not a very creative or fulfilling role to have, but it can mean decent airtime. Mac isn't on enough to play that anymore, and the show seems to be commited to the gastropub set.