Another vote for over-reacting and to let it go. Even the fussy high-end hotels have guests who know they have to pre-register any visitors (or Asian GFs/spouses, for that matter). In some cases it's truly BF/GF or husband/wife, whereas in others it's a more temporary type relationship.

The really difficult/insulting one would be if the woman wanted to go out by herself in the morning for, say, breakfast, and the hotel security stopped her at the door. Many hotels won't let women out who look like locals or who may have been temporary visitors because of problems with drugging/robberies (the "reverse Cosby"...guys are victims more often than perps), so they'll make the woman give the room number and call up to the guy and ask if it's OK to let the girl go. Which isn't exactly compatible with being registered to your own room (though if you "look" foreign they won't do this to you). Hotels would rather get the mistaken identity and insult complaints than the bad PR from guests being robbed/drugged (or even found dead) because of visitors. It can be a bad predicament.