Here is how WW and I have decided to go forward with a way of establishing NC with one of her APs and make sure that the relevant OBS knows in this case. WW sent this note to AP and me.
I should point out that this letter is the product of a compromise between WW and me. I'm glad she's decided to work with me on this since she's been very clear in saying that she'd prefer that OBS never know at all. It has meant that I've conceded a little bit of ground in order to work collaboratively on this, but I'm basically satisfied that we're moving in the right direction here.
Thanks again for all the input about this earlier. Feel free to critique and offer predictions, cautions, pointers, advice, hopeful remarks, etc.
*****
Dear AP,
[My BH] recently learned about the nature of our relationship, as well as other infidelities on my part. As part of our reconciliation efforts, I cannot be in contact with you any longer. If you attempt to contact me individually via email, Facebook, text, or phone, I will not respond. In the possible case of us attending the same social function at some point in the future, I will be civil but do not plan on having any interactions or conversations with you. Another aspect of my reconciliation with [BH] includes transparency on my part. As such, I will share any instances of contact between you and me, either technologically or in person, to help rebuild trust with him.
The reason that [my BH] is included in this specific conversation is because he feels very strongly that [OBS] has a compelling right to know about our infidelities. He and I do not entirely agree that this implies a moral obligation on his part to make sure that she knows, but I believe he is sincere when he says that it is something that he needs to do as part of his process of healing from the pain that my infidelity with you has caused him.
[BH] and I both believe that it would be better for [OBS] to hear it from you and that there will be a higher possibility for reconciliation (and a stronger marriage) between you two than if she hears about it from anyone else. In fact, no matter how [OBS] finds out about our infidelities, in light of my own experience with [BH], it is now clear to me that you will not be able to have a healthy marriage unless you are able to be truthful with [OBS].
In any case, if you decide to tell [OBS] yourself, you will need to research and determine the best way to do so, though [BH] and I believe that you should at least be clear on these three points: (a) that our relationship was not Platonic, (b) that this aspect of our relationship began before you were married, and (c) that this aspect of our relationship was ongoing for a year after your marriage. But, no matter how you decide to go about telling [OBS] or what you specifically decide to tell her about the infidelity, [BH] will need confirmation that you have given [OBS] these basic facts of the situation. There are a number of ways that this confirmation might be accomplished, but I believe the best way for this to happen is for [OBS] directly to contact me, [BH], or both [BH] and me (your [BW's] preference) in order to confirm that you have told her about the infidelities between us. We are uncomfortable with the fact that this puts a burden on [OBS] to contact one or both of us, but it is the only way we can think of that allows you the opportunity to tell her yourself and for us to still feel confident that she is aware of the information.
This should be done soon, and we expect to hear from [OBS] by December 7. If we do not hear from [OBS] by that date, [BH] will feel obligated to contact her himself and supply her with evidence of the affair.
As far as your [BW] is concerned, [BH] and I both believe that she should be encouraged to do whatever she believes will best help her heal from the hurt she will feel. We are aware that there is some likelihood that she will respond angrily toward you, toward me, and perhaps even toward [BH] because of the pain that this revelation will cause. In light of this, if [OBS] would like any additional information or confirmation from either [BH] or me about the basic nature of our infidelity, she should feel free to communicate with either or both of us via Facebook.
Thank you for your understanding,
[WW]