How do I tell a woman her husband is trying to sleep with me?

Okay... long story short, this man has a history of cheating. He has a current wife and they have a son together. I’ve never met her and we don’t have mutual friends.

He is trying to hook up with me.

I’m going to tell her but unsure as to how to approach this subject. Obviously it’s going to be hard for her, so I want to do whatever I can to make it easier for her, without compromising the believability. I also don’t want to put myself in danger.

What should I do???