There's probably a problem in the route between your server and his server. There's not much you can do... he could try a few things like pinging your IP server from his box, and if it replies, edit his resolv.conf so his box can find your domain properly.
Well, of course first he really must be willing to help you. I wouldn't expect this from a site owner just because it can be a lot of work, specially if several people want him to do it too.
