I have all the proofs, but she denies it. Am I losing my mind?  

Incel. Men statistically cheat twice as much with women generally investing more in the relationship and children. Incels like you blame women so men don't have to take accountability if their bad partners or fathers.

I think is because that situation hit him and not another work related person. He already know she was cheating but he couldn't accept that

doing things for personal reasons always makes the job harder. it clouds judgement and such instead of being neutral for someone else's problems. it's like how therapists can't be their own therapist and they certainly can't be a loved one's, either