Yes Eric, the guy was an employee. I have no clue whether he was commuting, deadheading, traveling for fun, etc. Either way, he wasn't a pilot (based on his bags), so I assume he was a ground agent of some sort or worked in another department.
I'm thinking the gate agent upgraded him instead of me (the whole "taking care of our own" mentality), and thereby didn't go by the upgrade priority list.
I'm not sure why he didn't get back on the plane. The flight was sold out, but I figured we would have at least swapped seats, but instead they put someone else in my old seat.
My guess would be that this was a JFK ground agent that was flying non-rev and the agents confirmed him for the flight earlier than they should have, given that they didn't know that the earlier flight would be delayed. Then when this guy realized I'd probably be writing to United he went to talk to the GA, and the GA probably said he was out of luck since he was a non-rev. That's just a guess.
The only thing I know with (near) certainty is that he should *not* have been in first class until all upgrades were cleared (per United policy).