I think that not inviting Larry's GF would only be rude if they were an established couple - for instance, cohabiting, married or engaged. And in that scenario, if GF wasn't invited, Larry could, I think, if he had a close relationship with Aunt, ask her whether she could include . However, if she said no, or if his relationship with her wasn't close enough that asking would be OK, then his *only* polite options would be to attend alone, or to decline the invitation.
Pushing it, and in particular turning up with an uninvited was staggeringly rude.
I do agree that if Larry was the only guest not to have a spouse or long-term partner and therefore the only one invited solo then it would be gracious of aunt to make an exception and invite his GF, but given that the invitation was clear from the outset, she would not be rude not to invite GF.
I don't think that Aunt in any way highjacked the holiday or failed to host properly - she invited the people she wanted and could manage.