For any of these tricks to work, you need to be using DreamHost’s DNS for TheGreatModel8.net.
If he wants to maintain it, create a new FTP user here and give him the password. Then grab his stuff and drop it here, then go to the registrar for TheGreatModel8.net and set the DNS to use DreamHost’s name servers.
If you want to leave it as a subdomain of remingtonsociety, again, it’s best if it’s its own FTP user, which is easy to do in the Panel; just Edit that subdomain and change the owner with the Move Files option.
With the DreamHost DNS option, you can Mirror thegreatmodel8.remingtonsociety.com so it shows up as thegreatmodel8.net and it will work like magic. No extra work required.