I think the suggestion to just go ahead and have him there for the talk is a good one. I'll suggest that to him.
Honestly, I really don't think I've done anything wrong nor do I believe that I could convince her that I'm not a "cowgirl" (interesting term, never heard that one before). I'm going to state my case and then leave it at that. There is no point in engaging in an argument or getting defensive over things... although I'm worried that is what she will try to do.
He told me that she's nervous about this talk too, and that she told him that she thinks it's going to be a verbal beat-down session on her, which is like the exact opposite of my intent. I don't know what to make of that statement but it doesn't help me feel any better about the upcoming chat.