Can i ask if you have ever sorted mail before? Mistakes can happen. It could easily have been misread by the postman as an honest mistake.
As for what Stuart should do with it. He should stop being a miserable fucker, write on the envelope was delivered to the wrong address and re-post it. Posties are not allowed to take mail from individuals, and they must post it through your letterbox. If you don't like the rules, tough shit they are there for a reason.