He won't have to tell the insurance company about it. As soon as they see that seat after an accident, they'll void the policy as improper equipment. The most that might get paid is liability for the other party, but forget about any medical bill coverage from either side's insurance company.
But, as everyone else says, that'll be the bus owner's problem. What will be interesting is if someone else drives the bus and is injured. That's where we'll (well, he'll) start hearing terms like "criminal liability."
Mind you, I'm a fan of repurposing things, doing your own thing and saving a few bucks, but there are some places where you spend the money and use what's correct. A seat on a moving vehicle is definitely one of those places. That seat would be better used somewhere else in the bus, like as extra seating when parked. Office seats have a typical weight limit of 250lbs, and they will break if a 300lb person uses them. ****, I'm 225lbs and those 250lb seats make sounds quite a lot like a submarine hitting the red line on the depth gauge if I sit down in them too hard.
