I don't know if there was a seat for sale, but my friend did say that First was available according to the gate monitor, and the seatmap showed 2 open seats. He doesn't think there were enough eligible passengers, which is why he thought it was odd how they didn't just put him up there when rebooking. When they rebooked him on the IAD-DEN flight, the flight was already delayed, which is why the UG list was already processed by the time he was rebooked onto it.