What has been said here and by Dr. Harley is children should always be told, even if the affair was 20 yrs ago. I need to tell them, who else should be told? Full list? Just family and parents?
I would keep this within a close circle of family and friends, in addition to contacting the OM's GF and the OM. I would send something to this effect to family and friends:
Dear friends and family,
I am writing you this message because you are an important person in the lives of xxxx and I. I am asking for your support in our marriage. A few years ago, I discovered an affair between WW and her old boyfriend, OM. I was devastated. This affair consisted of hundreds of messages of very sexual, personal content. They may or may not have met up at the time. I know it was planned and don't know if they followed through. Contact has been intermittent recently and WW has refused to end contact.
I want our marriage to recover from this affair. However, that cannot happen as long as this guy is hanging around my marriage waiting for an opening. I live a life waiting for the other shoe to drop. If you have any influence on my babe, please do what you can to get her to stop this affair once and for all. I want to stay married, but this must end.
As our friends and family, I am asking that you use your influence with xxxx to persuade her to end her affair and try to work on our marriage. Our marriage can be salvaged if she would only end this for life. Please support her in doing the right thing. Please support our marriage.
I would so appreciate your support and prayers. Thank you, Jaded Husband
Then forward your email to the OM's parents and his girlfriend, ccing that dirtbag:
Dear Mr and Mrs OM and SallyGF, I am forwarding you the email I just sent to our family and friends about the affair between OM and my wife. I felt strongly you should know about the affair since it has never ended. I am asking that you use your influence to persuade your son to stay away from my life forever. His interference in my marriage for the past XX years has been devastating to me and my children. Please ask him to stay away.
Thank you, Jaded Husband