Laurie,
Once long ago, I was approached by someone I trusted who then shared with me something about a loved one of mine, something that was spoken in confidence and it was bad news. It upset me to hear it, it upset me that the person told me and it upset me because it destroyed any bond of trust that I once had for the man who told me the bad news.
Having this new information did not change my course of action, didn't change or help the situation one bit-except to cause me extreme pain about something I had no control of.
These days I try my best to stop a person mid -sentence or even mid- syllable if I think their words are harmful. It's a boundary I work hard to keep. I'll admit that what your former boss did made me very angry- somewhat because it was a trigger to my own experience but mostly that a professional person violated the trust of a client.