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.
Why should he give BTC back just because they know his address and a name?
When you look at their TOS, I don't see on what ground they demand the BTC back, as they say that "
Orders, once executed, are final and irreversible." So, if orders are indeed irreversible once executed and if PDAX is just an exchange between the two users, how come they ask money back? Then again, we heard just one side of the story as he didn't share what exactly PDAX told him is the reason for wanting the BTC back so there might be more than he is telling us.
Anyway, strange behavior from an exchange and if it's true what that guy is saying, I would contact the lawyer and let him deal with them.