Which is worse? "A" lying about "B" in the public forum, or "B" publicly publishing private messages to clear their name of "A's" lies?

This a terrible situation to be in. While someone might feel the need to post a story, question or comment when they are hurting - no one should ever name names or hatefully out another. There is no good answer to your dilemma. So do the best you can, don't do anything to cause yourself to feel bad later on, and remember...the people who know you, already know what kind of decent person you are.

This is tough. I'm my opinion the man can defend himself, reference the private messages in said defense, but doesn't need to go out of his way to post them. At that point, it's just stooping down to her level.

Hmm that's a hard one.

I am almost never for the breach of confidentiality aka sharing private convos. Yet on the other hand I also despise defamation.

He could defend himself without revealing the convos, yes?

Based on the hypothetical details presented, what Turtlepower said expresses my own thoughts better than I probably could have.

Telling lies about someone is absolutely contemptable, always. Doing so in a public forum, sure does put one in the position of wanting to defend themselves, and I know I would want to.
The thing is though, "B" could only prove what is claimed about "A" in this scenario, by posting ALL private messages between both parties.

When people spread lies about other people it may tarnish their reputation forever. I think that alone is worse. I really don’t blame the man for trying to defend himself in that case.

C - Reading this post and thinking of a response...

There's 3 sides to the story; his, hers and the truth. He could choose whatever bits & pieces he wants to make her appear batshit crazy, it doesn't mean she is. Hell, he could've been the one to drive her crazy if so and chances are he's likely married himself, but withholding his own secrets.

They both despicable.
A is childish immature tryin to make themselves look better
B is lowest of the low if revealing private Dm's