TLDR: Should I tell someone's long term mistress he's been cheating on her for 18 years? (Am already going to tell the wife, who he's been cheating on for 18 of the 20 years he's been with the mistress and been cheating on the mistress most of the time as well.)
More details: I recently discovered that someone I was in a relationship with for a year turned out to be married. He and I both traveled internationally a lot for work and he had an apartment in the city (the wife and kids were in the home in the suburb, which is why I didn't figure out the truth for awhile) so it wasn't unusual for us to be away from each other for days and weeks and have odd hours. I discovered his wife and family when she commented on a LinkedIn post referring to him as a great dad. When I dug around, I figured out he was married.
I ended things with him immediately and as a final breakup, he agreed to tell me the truth about everything he'd been lying about. This truth included that in addition to his wife, he also had a coworker he was in a committed relationship with for 18 years! So for 18 of the 20 years he's been married, he's been cheating on his wife with his coworker and for the last year, he's been cheating on the longterm mistress coworker with me. He also admitted to having standing relationships with two other women the entire time he was with the mistress. The mistress knows about his wife, but not me or the other women. His wife knows nothing at all.
The longterm mistress has never moved on or had a relationship outside of him because he keeps telling her he loves her but can't leave his wife. After learning all this, I'm pretty decided on telling his wife via anonymous means.
But I'm also considering telling the longterm mistress as well. I get that conventional wisdom says the mistress is a PoS for being with a married man so she kind of gets what she deserves. But when he found out I knew about the wife, he was VERY convincing in trying to talk me into staying in the relationship. If I weren't older and wiser already, I might have fallen for it. He started up with the mistress when she was 22 and he was 38 so I think age and maturity was a factor too. Of course, I think she's behaved badly as well, but I also know how convincing he was with me and can see how he's been stringing her along.
WIBTA for telling the longterm mistress who put her life on hold because he promised her he doesn't love his wife and only loves her but can't leave because of the kids when in reality, he's been cheating on her for almost the entire 18 years she's been waiting for him?