“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.
In keeping with their policy, it looks like he will get his money back, so he is fine.
I was more or less concerned about the airline using a "added value" excuse in the future to peanlize customers for unsold tickets.
The only reason this popped up to me was during a recent flight when the stewerdess and me were joking about how I had only one ticket, but the row was vacant, so I had 3 seats and one day the airline would find a way to make me pay.