Couldn't he just say good-night to her the way you would say good-night to a normal person?
He could, but she's not a normal person, she's a stunningly beautiful and charming woman who is his girlfriends best friend. If anything he's emphasising his politness so it CAN'T be misconstrued.
Instead of attempting the chivalrous gesture as the flipside to all the boorish talk about sex his friends bring along?
Because they don't bring it along, they keep their comments to themselves.
Reread the ONE scene we saw where they meet Kory, and the only comments they make are internal throughs, and if we went around judging people by what's going inside their heads, the world we would be severely depopulated inside a week.
They only really act in a debuached fashion at the stag do, and that's so much par for the course, it's ridiculous, Candy is professional enough to even make a joke with Dick and Barry (Who are anything but boorish in the scene) about it.