My (26/M) girlfriend (25/F) got into a fight with my best friend?s girlfriend. Idk how to handle social gatherings now.

It was not a fist fight just a heated argument, but my friend?s girlfriend said something offensive to my girlfriend (not going to share this info cause it?s private and she would be very upset). They hate each other at this point and used to be friends but haven?t spoken for 4 months now. A line was crossed, and the friendship is damaged beyond repair.
My best friend invited me (and only me) to his birthday that his girlfriend will be at. I understand that he doesn?t want it to be awkward for his girlfriend but now my girlfriend isn?t welcome to these gatherings and it bothers both me and her. All my other friends? S/Os are welcome. I don?t want to be showing up solo to these events.
I?m thinking of having a talk with my friend about not going, and explaining that our friendship dynamic will have to stick specifically to ?boys nights? at this point. I?d like to offer to take him out for food and drinks to celebrate in a different environment. I?ve told another friend of mine this and he thinks I?m handling this the wrong way.
My girlfriend is feeling thrown aside (and has cried a lot about it) and I want to stick up for her. Their argument was a result of her sticking up for herself, and I would be a ****ty boyfriend to leave her behind as I hang out with people who treat her poorly. I think I can still maintain my friendship with my best friend by setting these boundaries, even if it means severely limiting the social settings I take part in.
Does this seem reasonable?
tl;dr My girlfriend got in a fight with my best friends girlfriend and now she?s uninvited places. I say we are a package deal.


Source.