You have sympathy from me @Ladywit
It's too easy for other people to condemn anyone who does something they disapprove of.
I know several couples who were married when they met each other. These include couples who left long marriages when they were in their 50s and 60s. It happens all the time. It's life. It's not ideal, but what is? As you say, it's when people meet someone else who seems to be all they want that it gives them the courage to leave.
What's sad is when it doesn't go ahead as 'planned' - ie one party chickens out of leaving the marriage.
People are terrified of change. They hang on in unhappy marriages for decades for all kinds of reasons.
That, imo can be as bad as meeting someone else and leaving- it is still a kind of deceit - to stay with someone as it's 'easier' than the upheaval of divorce.
But I do think you want his wife to know a) as revenge and b) in the hope he will leave her for you.
He will certainly despise you if you tell her. He may 'deserve' to be outed, but what good will come of it?
You would feel better for a few hours, or days, but then you might feel a lot worse.
I suspect if this was a long affair that his wife knew anyway- or certainly had an idea of it.
I think all you can do is admit you took a risk. Anyone who waits for a married person to leave, is taking a risk. Sometimes it works out and two empty marriages are ended, the people involved on all sides move on and find happiness, but sometimes it doesn't end that way.
Accept you took a risk and you lost.Maybe you can console yourself a little by understanding this woman has her H, though for how long who knows.
You did the right thing to end your own marriage if it was unhappy. That's a good outcome. The married man was always going to be an 'if'.
He is either not as unhappy with her as he appeared or he doesn't have the guts to be honest with her and himself, or there are children and he is putting them first.
Take some time to heal. It might take you a year or more. Be kind to yourself. Go out, make friends, cry, scream, do whatever you need to and don't look back. This has been a shockingly painful experience but you will come out stronger for it.