It is comforting to see some have noticed that there is a legal void in who maybe responsible and that may be a big issue if things go real bad one day.
Just to be clear
Band a Lot, in the case in question, UAL and the Doctor in Chicago, this was entirely a seating matter for the ground staff to resolve and the captain was not and should not have been involved. Had the ground staff chosen to do so they had several layers of management they could have referred to and the upper layers of that management stream have significantly more authority than the captain. My personal view is that only one layer above the person who approached the doctor would have said, "up the offer and ask some other passengers". Quite why one crew member of the DH crew could not accept a jump seat for a 45 minute journey I'll never understand.