I agree, ass-hole moves all around.
The dumbest part is that she was out with friends already having a good time. It's not like she was moping around the house feeling sorry for herself.
My girlfriend's husband's boyfriend always picks his wife up from work and drives her home. If he happens to be visiting at the time, he'll leave to go pick her up, drop her off at home, and then come back over. That's just their arrangement, and it works for everyone.
I suspect there's more to it than the wife just needed a ride home; after all, she didn't ask him to pick her up at the end of the night, she asked him to come join her part way through and stay with her. She was with friends, and surely they had to be getting home somehow. She could have hitched a ride with them. But as was mentioned, none of that has anything to do with you. Your place is to communicate with your boyfriend how you feel about him leaving your date to go be with her, and how you cannot be in a relationship with those terms. Make it clear that this is about you and your needs, not about the wife's behaviour.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker