I'd be mad, yes. Why? Because girl politics are confusing. Would I understand that he was not aware of girl politics? Yes. Would I understand that she was aware of them and was using him to get under my skin? Yes. So I'd be mad. But I wouldn't expect him to understand.
Excuse the language. But I strongly agree with the saying "if she doesn't fuck with them I don't/ if he doesn't duck with them I don't" ( unless you were already long time friends that's a touchy subject) but that fact you were aware of your girls dislike for that particular girl. You were wrong and leaving her alone for 20 min to speak with her was also wrong.