Just go "Oh, I am not with this airline just a passenger, but you need a belt extender do you? Shall I ask for one anyway?" If yes.. "HEY STEWARDESS THIS GUY IS TOO FAT AND NEEDS A SEATBELT EXTENDER GET ONE PLEASE? SOME HELP WITH THE FAT GUY HEY RIGHT HERE!"  Be polite, help the poor guy out anyway.