So I accidentally stumbled in here and I'm making some broad and very possibly weong assumptions that most everyone else in this story is allistic. If that isn't true you can probably ignore this, my bad.
That said OP if your GF is allistic, and especially if this is a newer relationship, you may need to tell her that she needs to clarify her expectations explicitly with you in this area. If she is allistic there is no chance she'll have thought she had to ask you "please don't abandon me on my birthday to go on a classic date with someone other than me." To an allistic bystander this whole story reads like you care more about this friend than your girlfriend who you dont seem to really like all that much. (Not saying it's true just that that is gonna be the social impression given off). Also, if you didn't do something romantic still leaving your partners party to go hang out with a friend will still largely be interpreted as prioritizing that friend over your girlfriend. I'm not sure if that's what you were trying to do but that's what allistic people who hear this story will assume. So if she was OK with you leaving the party to hang out with this friend nonromantically (which seems unlikely) you may want to include "my GF told me she was fine with me leaving her party" when telling this story. Otherwise I think you focusing in on just the romance part may be missing the point, the romance part just made it worse but wasn't the whole problem.
Also as a side note: if the friend who asked is of a gender you are interested in and knew it was your girlfriends birthday when extending this invite (and is allistic) then your friend was knowingly being an asshole to your girlfriend. Possibly also trying to sabotage your relationship. Just a heads up.