Adults form social units and others do not get to judge how significant someone else's significant other is. I think the "rule" was rude to begin with and she should have known better.
Actually, yes they do. That's why etiquette has the social unit rule. Married, engaged and (since late 20th century) cohabiting. Otherwise, every single social invitation would have to have an "and guest" attached to it.
If Larry feels his love is more burning than a million suns, and he can't bear to be apart from his GF for a few hours, he can turn down the invitation.
The traditional rule here is that people can say "this is a significant relationship
" by marrying/announcing an engagement (which just about everyone will accept); living together and making it clear that this is a romantic partnership, not non-romantic housemates (which many people will accept as creating a social unit, but not all; or by some version of "Marisol and I are serious about each other, and want to be treated as a couple even though we're not living together," which, again, some people will accept, but maybe fewer.
The only one of those where traditional etiquette demands that even the newest of partners be welcomed is marriage (even if you're convinced that the marriage can't last, because the couple married ten days after they met). Personally, I would wonder about the level of commitment of that marriage, and would privately take it less seriously than I do some long-term unmarried couples: but if you're playing by the old/formal rules even a little bit, it counts unless and until they split up. And those of us who are asking for acceptance of nonstandard relationships
are usually also going to accept the standard ones: I may think it was silly of someone to get married after two weeks, but I also see that it's a pretty clear way of declaring themselves a serious couple.