I don't think he has no choice but to return it just like what you have said they know his address and name. But I think it's the exchange's fault and not the user the good thing to do is to maybe give the user a reward for returning the Bitcoin but if the buyer becomes greedy well I'm not sure what will be the PDAX's move after that.
It can be disputed when a legal matter is pushed forward. In my opinion, it was the fault of the exchange and they have to compensate the person, if I were in that situation, I think that I wouldn't return it or at the least before buying that, I would take a screenshot of that buy order, the person was right in every aspect and I think that companies that makes mistakes like this should face the consequences of their mistake.