The master server hasn't been moderated in ages aside from a few extreme cases where someone contacted an admin directly about it, and we're not even sure the flagging system works. Are you sure it isn't just something like your network IP changing causing the port forwarding rules set on your router to go to the wrong computer?
Are you absolutely sure he's flagged? Freezing when he tries to host sounds a bit unusual, and anyhow flagging should only affect appearing on the master server (to my knowledge)?
Also, has he tried hosting an IP-only server at all?