I wouldn't let it get to me. I'd do that too. But I think the majority of rational people who have studied this site and really looked into the dynamics of infidelity know different. The last thing you need is to get into a raging battle with OM's W. Let it go, if you can, and continue concentrating on the things that are really important in your life. If anything, feel compassion for her.
Love is meant to heal.
Love is meant to renew. Love is meant to oust all fear. Love is meant to harmonize differences. Love is meant to bring us closer to God. In my case, I would have understood him seeking sex outside the marriage more than I could ever comprehend my best friend looking for another best friend.