It appears that Southwest is more generous then I would be -
http://www.southwest.com/travel_center/cos_qa.html
IF they have not filled the flight, AND you have purchased the extra ticket beforehand, they will refund the extra ticket.
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.