Honestly, the OP will not pay attention to the letter. I'm sure they will think it is a ploy.
The OW in my situation was a WAS also. Not only did my xh ruin our family but she ruined hers...and for what? Not a damn thing as they are not together anymore. My xh has moved on to a new woman and left her behind. Did she take it lightly? Nope. She stalks him and harrassed him to the point he had to get his cell # changed. He had to give up his precious volleyball that he played 7 nights a week (with her of course) and everything else he enjoyed just to stay away from her.
I find it rather humorous now. I see her at times and want to say things to her but I am the bigger person and I keep my mouth shut. At our daughters graduation I was standing next to xh and she walked by. I know it kills her to see him and I together because she thinks if he can still be friends with me then he can be with her too. Of course I did move a little closer to xh and whisper something in his ear just to piss her off. (I said I was the bigger person, not that I was made of steel) LoL!
I think writing the letter is just a waste of time. Let them walk away with our spouses and deal with the backlash that I am sure will be in their lives sooner or later. They will pay the price one day.