I have no personal experience, but I've read the posts of others BS on this website who were in open marriages. For them the betrayal is very real and painful, just like it is for those who are in "more conventional" relationships. So in that sense I would guess the prevailing wisdom that I hear most often on SI (that the OBS has a right to know the truth) would apply regardless of the fact that they were in an open M.
I would just try to stick to the facts as you know them. If you have proof I would include it. If not, maybe give dates or details so the OBS could independently confirm the A.
There is a website you can use to anonymously inform the OBS, and it even allows you to attach proof (though I have not used it myself so I can't give any feedback):
Credit to befuzzled110 for the site reference which was listed in their post in the Investigative Tips forum in October.