@shan_dyl: A wedding-board meme has recently reached critical mass, that “etiquette says” anyone in any sort of a relationship must be invited as a couple, because it is not up to you to judge the “significance” of other people’s “others”. Some people, we are told, know that their squeeze-de-jour is “the one” within weeks, even hours, of meeting one another.
Cool. I am happy for them.
But let us all admit that “married” is a special kind of significance. These are, after all, wedding boards that we are all pontificating upon. If being married were not an extraordinarily big deal, then why would the wedding industry contribute forty billion dollars per annum to the gross domestic product of the United States, and why would so many young women spend years on these boards planning every detail?
So the rule — for all formal parties, weddings included — is that couples who have established a publicly-announced (or publicly demonstrated) marital status must be treated as a social unit. That includes married couples, couples who have agreed to become married (id est, engaged couples), and couples who are living together as married without benefit of sacrament or civil registration (id est, common-law couples). And that, not the wedding-board consensus or the wishful thinking of happy-looking couples, is what etiquette actually says.
But people make choices. Guests can choose to ignore etiquette, and reply for more people than were invited; hostesses can choose to stick to their guns; and guests can choose to decline invitations. I must say that I think a good deal less of a lady who puts a recently acquired boyfriend over a lifelong girl-friend, and I imagine you probably think less of her now, too. But fortunately, you have a couple more choices.
My choice, when confronted by someone taking liberties reserved for a people in a particular situation, is to assume that they are in that situation. When young girl friend starts talking about “us” in response to an invitation that was given to “her”, I sparkle at her and ask “oh, is he the one?!!! How exciting! Have you announced your engagement yet? Why don’t we announce it at dinner … I would love to offer a toast to the two of you!!!” After all, if he really is her “everything”, then why are they not ready to publicly declare their permanent commitment?
Your choice, since you are doubtlessly nicer and less manipulative than I, is to invite the boyfriend. NOT give Miss Old-good-friend a “plus one”; plus-ones are a violation of etiquette in the first place. But instead, get Miss Old-good-friend to introduce you to this wonderful man; have coffee together or have the two of them over to dinner, and then invite him as a person in his own right. You do not then have to justify to other people why she “got a plus-one” and they didn’t. You invited Mr Wonderful, because having met him you really do see how wonderful he is and have formed a friendship with him. And you as hostess — you are the hostess, right? Or else this is someone else’s problem to deal with — you as hostess can choose to invite whichever friends you wish.