Thanks to everybody for responding.
I basically sent him away on the grounds that he was cheating on his wife.
Still, I have this uneasy feeling about myself now.
I feel it's patronizing to tell someone how to live their life. And I feel I kind of placed myself above him, in a way, by implying: "I'm superior because I'm honest and you're a cheater". And I don't like that...
Well, just that. I guess this is no black or white subject.