I don't think there would be a right time to say that.

there is a time.. just not when meeting your in laws for the first time, its hanging out with your own friends Who would ser the fun part in it :)

I honestly don't understand why people just can't grasp when to keep their mouths shut. Not having a switch between "hanging out with friends behavior" and "visiting partner's parents behavior" is a huge issue. The only excuse for that is if the person has a mental disorder that makes appropriate social interactions hard to distinguish between.

Is OP his girlfriend or his mother? OP'S boyfriend should WANT to make a good impression with the parents, not have to promise to be on their best behavior before hand like a little kid going to a birthday party.

I think the dinner took place at her parent's house.

Well if that's the case, at least they got (most of?) a free meal. But who knows, they could be laughing about this story at their wedding reception in a few years

Ha I guess my family dinners with the in laws are much different than yours.