Maureen knows within seconds of arriving at dinner that something is going on with Serena and Trip, and she confronts Nate about it. He gives her the footage of the cuckolds making out in the elevator, which she then blackmails Trip with to stay married to her. Ah, romance.
I kind of feel for Nate, too — even though his feelings for S are totally misguided and boring. But when he pathetically asks S to give him a chance, I hoped her puppy-dog face meant she might.