This is why I do not date men with female besties. I don't care if the friendship goes back to childhood. It's not the woman I have an issue with, it's the man. I don't even have close male friends anymore. It is a hard boundary for me. The girlfriend was foul but I'm not going to say her paranoia was unfounded. Men know how to use their close friendships with other women as a way to create drama and paranoia in their own relationships to make the woman insecure and therefore more attached to them. I've been on the other side of that before as the close female friend who didn't realize what my male buddy was doing. It was easy to call the girl out as crazy until I saw the truth for myself. Any girlfriend in this situation will feel a little insecure when her BF's own father has said that he thought his son would marry the female bestie.
And the motivation for a woman to keep a man around as a friend is different from the other way around. Women friendzone dudes all of the time, but men don't usually keep close friendships with women they don't have feelings for. I believe this friend has feelings for you, and had you responded to his gf's shenanigans positively, and she called him out on it, you might have seen a different response from him.
This isn't to say that close male-female friendships can't be healthy, because I'm sure they can, but they don't exist in my world for a reason. You haven't done anything wrong but you have no way of knowing what seeds this male bestie is planting in his relationships. If her intuition led her to do something so ridiculous, there might be a reason.