This particular problem is quite common in this stage of your business. You have to cover legally all the data that might be leaked from some other places and w

This particular problem is quite common in this stage of your business. You have to cover legally all the data that might be leaked from some other places and when you are completely sure that there can't be any other source of leakage, ask him politely the return of that data to adhere to a higher standard of information security. This has to be a very delicate request. I have been both requested and requester, good luck dealing with this.