I didn't want him to confirm my opinion or call her a liar.

If I was in his shoes, or if I was training CRM to employees, the response would be something like "Sir, we are sorry, our employee made a mistake. There is no reason she should have repeatedly rung the bell at 8:45am, even if there was or wasn't a Privacy sign. We will make sure it doesn't happen again and will re-inforce these aspects thru further training." That would cover it. Maybe follow up with a note from the GM recognizing the problem and accompanied with flowers or a fruit plate in the room. (I did not and don't want that, but that would have been my "textbook solution".)

It is about taking ownership of a problem, and not making the customer even more aggrieved. Sadly, IMHO, this manager failed miserably this morning. And yes, his response did leave a rather bitter taste in my mouth.