"He knows but he won't admit it" -- obviously speculation. How do you know if he would admit it or not? The speculation was not about the affair, everyone knows that is true, the speculation was about whether or not Chris would testify to it.
She asks "why she would hire him is insane" and he responds "yes". Again, obviously speculation, he isn't a psychiatrist, he can't diagnose if someone is insane or not. Unethical or illegal, yes he is qualified to say that, but if we are talking about insanity you need a licensed professional, otherwise it is just speculation.
She asked him if he thought it started before she hired him and he said "absolutely". Again, pure speculation, you don't know how absolute the relationship was, especially since they are both adulterers. This could have simply been a fling. Maybe Nathan was simply sleeping his way to the top.
After giving a detailed account of where they met and when the relationship started he said "but you can't say this because he is one of the few people who knows this". Again, speculation, how would he know how many people knew that?