[Mithaldu]

They didn't "change" anything. That would mean it was a mistake of omission in informing him.

They stole his boarding pass, replaced it with another one, lied to him about sitting in the wrong seat. This was done in the hope he'd be stupid enough to think he misread his boarding pass.

I'm not sure how official such a document is, but he might've even be able to sue them for forgery or at least fraud.