Yes, it helps, please ask him to post a good evidence that he owns the address.
Before I put him into the trouble (I'm the one who made a mistake!) of signing a message with his address to prove the ownership I would like to know how this will make it possible to find the owner of that 1Nbjpo1tkeayJ28me2kNcZ3ZoqBQHJqoAx address. As you said, if this address would be yours, you would have sent those BTC back because you were "not entitled to receive 5 BTC from any source". If if turns out this address really belongs to bit-pay, I would like to see them to do the same thing. But I'm not sure if it's bit-pay. And I don't see how proofing ownership of the *source* address will help in this case. It's okay if those BTC get sent back to the source address. I will then ask the user to send those 5 BTC again, to address which I actually own.
All right, that is fair enough. You do not have to really prove that for me, you can reserve the evidence for the potential new owner of your 5 BTC.