She as the primary has the right to negotiate with him things she does not want him to to, generally. It is not like she goes to sexparties with him herself, but does not want his girlfriend to go with him!
He as an adult have the right to say; I don't agree with that. In this case he has agreed to something, then by defoult (not checking which party they would attend) breaking the rules. It is clear that he in reality does not value the agreement he himself made
. That is problematic in itself! Maybe he breaks the rule because it is not fair, or because it was hard to keep. Then it is his own responsablity to go to her and say; Ok. I broke the rule, I am sorry. Things seem to not work smoothly, should we go over our rules again? How can we make rules that are workable when I am with girlfriend? And - if ever - girlfriend is moving towards being more than secondary; what rules can apply then? With two primary girls, who would make the rules? would there be rules?
ps; I am in the process of getting a second primary, and that is not easy. It is like stepping into unknown waters. I have been used to having a sor-of-secondary for years and I imagined I would find another, I could not imagine even wanting to live with anyone but my husband. Life is scary, sometimes, it gives out challenges. The best of luck with yours