Should I let my former undergraduate advisor know that my PhD advisor was abusive?

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.

Yes. Abusive persons in positions of authority are a huge problem in academia. A major reason for the continued existence of this problem is the fact that it’s often swept under the carpet and hushed up.

Ask yourself: would you have liked to know before starting your PhD that your future adviser was known to be abusive? The answer, I hope, is self-evident.

Disclosing the abusiveness of your advisor might not help you personally but it would be a huge service to the academic community, and in particular to future students. Put in a different, more negative way, by not letting people know you’d be contributing to the problem. Unfortunately there are often good reasons for keeping such things to oneself (fear of reprisal probably being the main factor) but since you’ve moved on to industry, you’re in a unique position where disclosure is unlikely to cause you any harm.

One of the big revelations of the #MeToo movement is the fact that many people honestly don’t know that a colleague of theirs is a harasser, and thus are unable to help victims, or prevent abuse. The same is true for all forms of abuse, not just sexual harassment.

I agree that you should disclose this information. I know of a colleague who has a reputation for being hard on students, but I don't know whether it crosses the line. I know another who is a raging jerk, but again I don't know whether his annoying personality crosses into abusing students. In both cases it would be tremendously helpful to have a student tell me "Look, don't let anybody else get into the situation I wound up in."

Don't let them take advantage of fear. That's how the powerful get away with it.