I will try to be as succinct as possible here.
I have discovered that my H started seeing another woman approximately 10 years ago. They would meet on a regular basis during the day. Her place was within walking distance of his place of work. I never suspected a thing for 8 years. It developed into a love affair. He was going to leave me when DC turned 18. This never happened. I assume because second DC turning 18 coincided with first DC suddenly choosing to live abroad for a year. I am guessing that he couldn't bring himself to leave me at the same time first DC was going away for a year.
The affair ended. I assume because of H not leaving me, but don't know exactly what happened.
OW met a divorced man, married him, had a child with him, but the affair with H re-started at some stage during this time. Lockdown happened. H and OW obviously could not see each other although I didn't know she was OW at the time. H and I became much closer during lockdown. Unfortunately, this closeness was what led me to discover what I have discovered.
H does not yet know that I know the truth. I separated from H after he admitted infidelity with others to me. He has always strongly denied any involvement with this decade-long OW.
My main question at the moment is whether I should speak to OW's husband. I have never spoken to this man. I think he may well know that she and H were seeing each other years ago, and he would have known that H was married. Now they have children together, I am not sure about throwing a grenade. I know from his SM postings that his first wife (with whom he has children) left him for someone else and he posted that he went into a depression when this happened.
I have been through hell trying to get to the truth of this. All I wanted was for someone to tell me the truth. AFAIK OW's husband does not know that she was seeing H while seeing him at the same time. Is he entitled to the truth or should I keep quiet?