My guess is that his dislike stems from the fact that this lady only made things better for herself and her family, and not GP, or anyone else on the flight.

If everyone can't do it (bringing more luggage than will fit above you) then doing that seem like you're doing it because you're better than everyone else, which is offensive to most people for good reason. If her whole family had flown first class, that'd be a different story. (They'd probably still be annoyed at the family for basically cancelling out some of the perks of paying extra for flights, though.)