Hello Time. I dont normally post on wayward, but i kinda felt compelled.
I very much sympathize with your BS's feelings. I have had them myself. But, from my personal experience, i have to strongly caution against this letter. Sending it will open the door to contact, and i dont think it will have the desired effect your BS seeks.
The OW in my case lives right down the street. I see you work with your AP. While not identical, it is similar in the respect that there is very close, constant proximity to the AP. because of that, i can understand your BS's need to definitively and specifically make it abundantly clear that it is O.V.E.R. DONE. GO AWAY. I had that very same need. I almost had it obsessively.
But i had such contact with the OW's BH...and in retrospect, i think it opened the door for the AP's continued antagonizing of me and backfired. It just confirms to the AP that you are still thinking about her. It tells her that she still has head space. It tells her that she is still significant in some way. It gives her an "in" and lets her know that she still CAN get to you. And, based on your statement
One time in that period COW e-mailed me to tell me that "I was making its life miserable and this NC wasn't going to work."...initiating contact opens that door for her to antagonize...drive a wedge...create tension....create suspicion....interfere in any progress you and your BS are making. It lets her back in.
I have learned the hard way NC is truly best. Ignoring the OW really is the best revenge. She may want the attention, she may seek it...and sending the letter may give her the attention she so desperately seeks.
It is hard, it is so very hard.