The one other thing to consider is this: He's already got a seat - YOURS! Now you are going to potentially be doing exactly what he has done - plonk down in a seat somewhere, not knowing if it is already assigned or indeed free. Why should his ignorance cause you further hassle beyond the initial encountered at YOUR already reserved seat?

This type of thing wouldn't fly in certain countries in Europe where lots of stuff is reserved, especially trains during peak periods.