Use his/her overflow as a pillow - arm rest - blanket - and just smile, if there are no seats on board any complaint is futile, if there are seats they should move you or the large person to it.

The whole US thing charging double is discriminating, the airlines should provide the space to cater for passengers who are large.

Next thing you know charges will be made for vege and other special meals, the extra assistance given to disabled people like the blind, elderly who need special attention from the inflight staff and the list goes on.