How did you feel to be facilitating with the cheater? Where you aware of her presence? If so, why the guilty conscience now? If not, I'd fully expect you to be angry, and if I were you I would 'dob him in'.
However, realistically, what is to be achieved from this?
Are you annoyed more at the injustices of what he is doing (morally)? Or is it the fact you have been duped?
I feel sorry for her- and only her at this point. Without more information I can't really objectively understand.
Above all-- what really makes me sick is the morality of the 21st century. This is the real issue here, why do people feel the need to cheat? If they aren't happy leave there current partners and go for a person better suited to their needs. It isn't difficult at all....
The motive really is pure selfishness...it muddies the waters and people aren't prepared to let go of what they have for something else (which may or may not work out). Cowardice really.
Honestly, I am engaged to be married with a lovely partner, but if I whatsoever caught her cheating she'd be out the door.
The cruelty of it is unbelievable.
Maybe I have arrived to some unjustified misconceptions with you OP, however, this is because you freely admit he cheated with you. The minute he gets back with her its the 'victim' complex and you go from 'okay, I've cheated with him- I've gotten my rocks off' to 'he's a **** he played me'.
I think its equally as selfish to have this attitude because your basically saying its okay to cheat with him, but its not when he gets back with her. Fact is, he's cheating on her--you allowed him and accepted him to cheat with you- otherwise you'd have asked him to break up with her and started up something new with him.
Morality is a key word here. You need to re-address why your so hell-bent on getting some sort of revenge and outting him. Meanwhile, at same time, he does really need to be exposed. Problem is, your obviously going to have to take some responsibility for your hand in this. Irrespective of your status, you willingly conspired knowing he was already in a relationship and cheating.