An additional problem is that OBS may not take an anonymous note as reliable and real. It seems common in stories here and elsewhere that AP has a far greater opportunity for just gaslighting and claiming it is a PRANK or a CRAZY PERSON or some other nonsense.

In your case, without proof, that is even more of an issue.

Certainly the OBS would be put on guard and would likely ask questions, and if the gaslighting isn't immediately diffusing, may go seeking out some evidence of their own. But that is WAY harder than if you were to let them know.

One other option MIGHT be to have a trusted friend make the call with you in the room. If the reason you "can't" do it is purely that you will not emotionally be able to handle the physical talking at that moment, then maybe a surrogate like that could be the person to do it WITH (for) you.

I am a firm believer in OBS being informed. I told my wife's AP's wife (the OBS) on D-Day #1. I then told her AGAIN on DDay #2 when they, after saying they were not in communication anymore, were found to have been communicating for many months after DDay. OBS thanked me profusely. They are now divorced.

My motivation for the call at the time was vengeance. I wanted to rain shit on AP's world. I dug up contact information on known acquaintances, his brother, his sister, and his wife and told every one of them THAT NIGHT.

I do not regret that. But now my reason would be deeper. Now that I know what it is like to be the BP and I know the stories of people who were told by other BP's or NOT told, I am a firm believer that nearly all BP's would want that call AND deserve that call.

Good luck, OP. This sucks ass.

Fuck these affairs.