[haudraufwienix]

There are not size and shape restrictions to seat tickets. People are not baggage. He should not have to buy two.

I'm betting their is. All the aircraft I am endorsed to fly have restrictions on passenger weights, and have had to deny passengers flying as a result.

Imagine having a guy like that sitting behind you in an emergency landing[help]I doubt the seats are designed to restrain someone of that mass and he will end up crushing you as he is jolted forward, even a gear up landing would be made significantly more dangerous as a result.

And before people start saying if a plane crashes everyone will be dead anyway, remember most crashes happen at landing (well every crash is a landing, but you get what I mean) and involve around the same forces as most car crashes.