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.