I'm sizeable, but I can still fit into a single seat without encroaching on my neighbours. It's hard to know what an airline can be expected to do in advance - there's no mechanism for a passenger to tell them that they're a lard-arse after all. However, I do find it strange that on a full flight, this person could be shoe-horned into a single seat when their size would obviously inconvenience the person next to them. The person affected should have complained at the time - I would have either demanded an upgrade, or insisted that the person be offloaded. Isn't it a rule that you have to be able to fit into the seat and get the armrest down, otherwise you need to buy 2 seats? More to the point, why was this not addressed by ground staff before the flight boarded? Sounds like this person was obvious, from a visual assessment alone, to be pushing the boundaries. I seem to remember Southwest doing this on that Airline USA program a few years ago.