But the boss is such a square ass. I mean, he had her cold busted there, and all he could muster was a confrontation in "boss mode", forcing her into a really weak lie?
He coulda:
A) Stung her with some nasty barb, "If the wine is too cheap for you, why did you have to drink the whole bottle?" or any number of other good ones.
B) Wait for her to finish, then tap her on the shoulder and say, "We'll talk about this on Monday." -- nothing like a weekend to think about it.
C) Not do anything, then blindside her with it some time later. This is always satisfying.
I'll note that he is a jerk: he's the only one wearing a suit at the party, and while he's given his employees plastic champagne glasses, he's got a cobalt blue flute for himself.
In a situation like that, you don't go to the office christmas party.