Should I let my former undergraduate advisor know that my PhD advisor was abusive? 18 months into my PhD (after I had passed my quals), my advisor became so emotionally abusive and bullying that I had to take a semester off (I was already dealing with diagnosed depression). While I returned for six months, ultimately I was not able to make it work with my advisor (who went from bullying me to ignoring me) and have left the PhD program for industry. 

I've kept in infrequent contact with my former undergraduate advisor, with whom I conducted about a year of research. She recently ran into a close friend at a conference and asked about me and how my PhD was going. While the friend did not disclose the specifics, he disclosed how my advisor was holding me back by requiring experimental work after withdrawing support/funds, at which point she said that based on my current work, it would be sufficient enough for a PhD with her (and that I should finish my PhD remotely with her).

Once I formally resigned my position (I have not officially "withdrawn" from the program till October), I emailed  my former advisor to let her know of my decision. She has seemed eager to reconnect now that we are in the same geographic area (2 hr). I'm planning on meeting with her to catch up, but struggling with how much I should disclose about my departure from my program. She is not a frequent collaborator with my advisor (they wrote a book together some years ago), but they very much travel in the same circles, are on the same committees, etc. Should I tell her what happened? Or will this just lead to gossip that won't help me personally or professionally? Both advisors are still closely tied to industry, fwiw.
I'm sorry to hear about your experience, that sounds very distressing.

From what you've posted, it looks as if your undergrad advisor is sympathetic and thinks well of you. That being the case, you probably don't have much to lose by discussing it with her, especially if she asks about it.

It's also unlikely that you'll get much personal benefit from discussing it, other than a sympathetic ear (which is sometimes well worth having!) But it's quite possible that you'll be helping other students. If she is aware of this behaviour, then she's in a much better position to look out for other students who might run into the same problems with this guy.