So how should (ideally) a machine be set up to be able to use it stand-alone as well as on a corporate LAN? Was it fine before with her username on the Domain?

My favorite way to do this is:

- Build the machine yourself, and make sure the owner of that machine knows its local Administrator account password. Or sets that password themselves during setup.

- After you create a user account to join the domain (girlfriendsdomain\girlfriendsname), sign back in as local Administrator and use the user manager to add (girlfreindsdomain\girlfriendsname) as one of the members of the local machine's "administrators" group.

That way, when the computer is on the domain, the person still has admin rights on that box, but is not an admin on the network in general. And they sign in with (girlfriendsname) no matter what environment they're in so it all Just Works.