Wow, if I heard something like that I'd feel upset and betrayed. I definitely understand why it would bother you and why you would want to do something about it. If true, it is a clear cut violation of confidentiality and open him to disciplinary actions. Thing is, what if it is not true, or more in shades of grey than that? Then it gets complicated. And what you do gets complicated.

Also you seem to already have a complicated response. You say you have no interest in a confrontation, but at the same time want to write a letter. Maybe I'm misunderstanding what you mean by confrontation, but to me a letter would be a confrontation so it feels like you may have mixed feelings about things.

If you sent a letter, would you want a response? And what types of things would you want to say in the letter?

Letter writing is sometimes recommended to process things so it may be the thing to do. I would just want to feel more certain about what it was all about and what I wanted out of it.