If I remember the episode correctly, the flight was from St. Louis to NYC. But you are correct about implausibility and a major plot error -- when they are at the gate making the arrangements for this flight following the cancellation of their original flight, the GA tells them that there are only two available seats on the plane, one in first class and one in coach. Thus the reason they are split up. So the existence of that empty FC seat in the middle of the flight was a bit odd.

Back on topic, I probably would not have had the guts to say anything about the self-upgrader, for fear of reprisal once off the plane, but would have been irritated about it and watching every move of the FAs, maybe even trying to make eye contact, hoping the enfringement would have been discovered and rectified by them.

Susan