Rfolan5

Hi, I recently traded a halloween Haunter to another player, however the poke is now region locked in their pc. We both agreed to the trade, and they did not ask for anything in return however I offered them a larvitar they could use once they got to the sinnoh region. I hadn't realized that I caught the Halloween haunter in the sinnoh region when I traded to him, admirably my mistake. Is there any way to resolve this issue? He hasn't asked for anything in return however I have offered to give him some John Pokemon to help him. Silver Edit: Zoruami - Check your PC last slot