Do you need to approach him? Has he told you about this extra relationship or did you find out some other way?
If I were in your shoes and I discovered that my seemingly happily married friend had a sexual relationship with someone other than his wife, I would choose to be a listening ear for him. I would choose to be the opposite of judgmental. If he wanted, he could feel free to unburden himself in my presence, knowing that his thoughts and feelings are safe with me. And I would communicate in some way, shape or form that he has my love, come what may.
That is, if he approached me. I don’t know that I’d be so quick to approach him about something he hasn’t shared with me on his own. Then again, that would depend on the kind of relationship you have with him. It could be that in the case of you and this guy, you know that it would be perfectly ok with him for you to approach him about something you’ve heard about him and his marriage. I don’t know.