Please help ASAP

[gutwrenching]

Ok - you did it. And he denied it, no surprise. Chances are good that they will go deeper undercover. You are getting a lot of great advice here. I also would not expose at work, but I also wouldn't let him get away with his denial.

I wouldn't let him know "how much" you know, but I'd make it clear that you know a lot more than he thinks and that it is not only appropriate from a M point of view but that it is borderline unprofessional and that you think it needs to stop now.

You are strong, continue with the acquaintance. Time to turn his world upside down a little, make him worry about how much you know, make him worry about life without you, and make him deal with what he is doing.

The "its too late" is an all too common line. Ignore it. It is the affair, the guilt, and the WAS talking, nothing more. He doesn't know what he feels or wants right now. You will likely hear much worse before this is all done