Guys: Would you lie to your best friend (male) if you were having an affair.?

My best friend is having an affair. I have known for over a year. I have made comment to him about it and he denied it. I left it alone. That is what a true friend does. But, over the course of a year I am tired of listening to him lie to me. He tries to tell me how he would never have an affair because he would never risk losing his wife. Yet, I know he is. He tells me not to ditch him for a "broad" yet he has ditched hanging out with me for his mistress.