So this is actually really important context OP. If you've "overstepped" with this friend before then your GF may already feel a bit cheated on. For you to leave her birthday to specifically meet this particular friend given your background info? That's a pretty devastating blow to the trust in your relationship.
There is no socially acceptable way to tell your GF "I needed a break from you , on your birthday, so I went to hang out with a woman actively trying to get me to cheat on you with her." Because it definitely sounds like thats what this friend was trying to do, if she's allistic there is no world in which she wasn't trying to take you on a date. (I understand meeting your friend wasn't preplanned, that matters very little here.)
Honestly OP I know you said in another comment that it hurt you that your GFs friends told her to break up with you but I'm seriously surprised she didn't. This is gonna sound mean but I really just want to be as clear as possible, so I appologize for how blunt and rude Im gonna be in this next part. I just want to make sure you understand whats likely happening in you GFs social circle. Full disclosure, if I had the information from these comments and was your GFs friend I would be telling her, "He cheated on you on your birthday, and he'll do it again. You need to break up with him. He doesnt care about you. Hes more into his friend." And I'd be repeating it to her every single time you saw this particular friend. It's wild to me that you're not single right now. I don't know of a single allistic person who would read this story, with the added context, and not assume youre at least trying to cheat on your girlfriend. If you want this to have a chance with your GF given her history of being cheated on then you may need to put this friendship on the backburner.
On the flip side, it does sound like you might care more about this friend than your girlfriend. Given the sexual orientations that you've outlined elsewhere have you considered dating this friend instead of your GF? All three of you might be happier? Just food for thought.