I would say it's very much a sacking offence. The manager breached his customers' confidentiality in a very serious way. The other side in the dispute has absolutely no right whatsoever to look for info on 'Mary's' account, never mind be given it! The manager has messed up very, very badly. Professionally and morally. How many other times has he done this without the knowledge of other customers? Who is to say this will not happen to another customer in future. It's disgraceful, in all fairness. I would (and I'm sure others would also) be looking for heads to roll over this.