Honesty is the best policy. However, it is all in the approach. I would approach it like it is this person's reputation or a rumor and let your friend find out the truth. I would rather do that than see my friend through divorce court with kids in tow in the future.
Would not agree that the wife opened the door, so to speak (regarding the 3-way). I think if a person is the kind of person who wants to cheat and decides to do it, then that's all there is to it.
Not knowing any more about the circumstances, the 3-way could have come from a lot of pressure from the husband (coping with being faithful, but wanting to be promiscuous) or perhaps it was an attempt of the wife to reconcile her husband's promiscuous side and combine both worlds. I'm coming down on the husband because you mentioned that he has an extensive history of cheating.
haven't said a word as of yet. just not sure.....
my close guy friends say: "no way, old news, let it go..."
Also, the cheater is/was a guy is a cop so it would have to be anonymous or thru friend of friend
to avoid an retribution I reckon - and the woman he is dating is pretty high profile sunny local tv anchor. it's rife with drama. very torn. one last question. since there apparently was a 3-way with his wife, does that mean she opened the door to this stuff...
I believe it would be better if she found out now, rather than later.
Tell her. She may get mad at you, but your conscience will be clear.
I agree with Trial, the messenger often gets the beating.
I think you should tell her, but kind of in an open ended way. You could say " I care about you, and I wanted to make sure you knew that Bill (or whatever his name is) has kind of a reputation from before for cheating often on his wife. I just wanted you to be aware of this before you kind of hear it in gossip, that's his reputation."
She can choose to ignore it, she might already know and doesn't care. If she's smart, she'll stop seeing him. The old saying, A man who cheats on his wife will cheat on his wife. It's true.
If she is still in the process of decision-making, a hint would be appropriate that you overheard he was some kind of womanizer in a former life which does not imply that he still is. Sometimes the messenger gets a beating from both sides, though... be aware. If however they already are a unit and share good moments, let her find out for herself because distrust could destroy the good part of whatever relationship they have while she might be able to cope with this guys behaviour in her own way.