And you believed him? If he'd see another woman while separated from his wife, why do you assume he'd tell you the truth about breaking with her?
I would have asked him if he had decided that it was over between he and his wife. If he said yes, then I would have talked to him about making it final and stop betraying vows he had made.
If he said no, then I would have told him he was driving a wedge between he and his wife if they ever DID get back together. He would KNOW he cheated on his wife, and it would hang over them like a cloud.
I find that sins are also things that violate human nature, and that a discussion of what wrong behavior does to self-respect, to people you've vowed your life to, to your own self-image as an honest or dishonest person is more effective than talking about the peril to one's soul.
Terror works with children. I don't use it on adults.
Although I can understand your pragmatic, jaded, "seen-it-all-done-it-all" attitude towards this topic, I couldn't help but be reminded of a biblical passage at your mention of the word "terror" -- the words of St. Paul, where he talks about working out his salvation in "fear and trembling."
If you wont' answer my other question, at least tell me if you dispute these facts:
Fact 1: Cohabitating before marriage is a mortal sin.
Fact 2: A single mortal sin, unrepented, will damn someone to hell for all of eternity.