I'm still not sure what to think about Debra, but I think her tone here was inappropriate, but understandable in a way.
First off, it's the boardroom and Ben has shown he will lie and take credit for things he didn't do. I can see why she wanted to mark out the territory of what she contributed, and Nick put that into question. Having looked back at the footage we see, I don't see Ben having any involvement on the book negotiation. It was led and closed by Debra.
Now I don't think she handled it well, but I can understand getting angry and irritated when an unjustified comment is made. Particularly here where Sir Alan takes Nick's comments into serious consideration.
I recall seeing some of Nick's biases coming through before. Unfortunately, in trying to explain what happened and battle against the perceived bias, Debra dug a bigger hole by speaking in that way.