Just my opinion but the better parent is proven out by being the one to not put the kids in the middle. Even if he does tell them something not right, if you tell them he is a lyer you would only be continuing to put them in the middle.
I know thats not fair, but I've experienced that from my own parents and can honestly say 30 years later, my mother was the bigger person.
M: 43 W: 43 Married 6 yrs. T: 7 yrs. Son 20, 18, 17, 15 yrs. (w/ Autism), 12, 10