Instead of getting into specifics, would simply listing them as "someone I would never work with again" suffice? If enough people do this then the specifics don't really matter... the warning sign is clear.
This seems to avoid the defamation issue because it is a personal statement (about yourself / your thoughts) instead of a statement about them. They could still sue you for defamation, but hopefully you could get it thrown out early at a low cost?
I'm not a lawyer, but I would assume that even that could be grounds for someone to file a lawsuit. I would like to believe that a judge wouldn't let that get very far, but if the person tried to claim material losses due to (e.g.) not being able to find work "due to your comments" then you might have to sit out a trial or settle.
Not really. She left enough clues that people on HN could figure it all out pretty quickly. Seems like a good middle ground to exposing them but protecting yourself.