How have you formed the opinion that a typical bloke deliberately says the opposite of whatever point/argument you ? Have you not met many blokes, or are you missing the fact that there wouldn't be an argument if the person you were arguing with was in agreement with you ?
Your boyfriend is entitled to hold his own opinions, and that includes how he feels about other people you know. There is give and take in a relationship, but it ought not go as far as dictating how one's partner feels. I appreciate that forming an opinion from one meeting may seem a bit unfair, but first impressions are important. If, as I suspect, you have expressed the opinion that he is being unreasonable and should get to know her better before dismissing her, then that is as much as you can do and should leave it there.
I trust he doesn't keep bringing the subject of your friend up ?
As for what to tell her when you meet, I don't see it serves any purpose to make your friend aware of his animosity. It would just make her feel bad. And besides he may change his opinion in future, or he may avoid her and so not cause further friction anyway. Even if they do meet in the future, the onus on him is to remain polite.
Answer your friend's question truthfully but noncommittally; it is surely the diplomatic way to go.