My point isn't that your friend is wrong. It's that if you are an expert on the field, you will always be able to pick up mistakes that authors make. So trying to make your stories absolutely accurate is pointless - they just have to be good enough to carry a reader with a basic understanding of the period. MacDonald Fraser (who I admire hugely) is a good example of this.
No... he won't. Trust me.
Though it is mainly his disgust at earlier historians who seemed to ignore a lot of readily available primary sources in favour of merely quoting one secondary source which was biased. I think Cornwell has picked up on that source which means he is not really to blame for others not doing their jobs properly.
One of his comments has been around the way in which both sides (the French and the English) were adamant that the other side would win because they were 'better organised and equipped'.