In that case, since he bought the ticket (at the gate no less) that would have gone unsold, he should just ask for a refund.
Though I have a feeling, he is going to milk this for publicity.
My concern has nothing to do with him and his fatty ways, but on an airline, that might argue you are getting extra value from their unbought tickets (i.e. having empty seats next to you) and thus asking (forcing) you to either buy the empty seat, or switch seats.
“In that case, since he bought the ticket (at the gate no less) that would have gone unsold, ....”
Until the plane departs you really don’t know that the ticket is unsold since someone could come up at the last minute and buy one, at top price no less. Would you bet that he did not go up first as one of the people who “might have trouble boarding” ?
Buying the ticket in advance lets the airline know that they have another seat being used, so they will not attempt to sell it, AND would give him the advance discounted price should he have to pay for it.
I’m certain a normal person could do the same thing if they wanted to insure they had an empty seat next to them. Buy the extra ticket and refund it if the plane is not sold out.