I go back to my original line of thought in asking if there would be any reason that your W would be contacting the OM. Would she have any reason to discuss the "job" except when she is at the work site?
I know I seem old fashion to some, but I do not think it is appropriate to talk to people of the opposite sex when it is kept secret from your S or it doesn't include your S. Something is very wrong with that picture. If it is job related, then it should be full transparent to the S and never covered up. But since you have talked to your W about this and she is "suppose" to have stopped, then do you think it's right or appropriate for her to be talking to OM "after hours"?
Again, I want you to take your time to get any proof of an A before you confront your W or tell OM's W. One call is not enough....even if it shouts A. She would wiggle her way out of it. I thought you had found some emails. Maybe I've got my stories confused...
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!