If you think that you can do it, I don't know because it seems OP has exerted all efforts to recover it, then offer him to do an escrow and ask him a proof of funds, my only concern here is he really doesn't own that Gmail, anyone can have a Gmail id and password but once you opened it in a different location and with a different device Gmail will trigger a verification if the one who opens it is the real owner, if there is valuable info on that email the owner, should have secured it, in case of a breach of loss.