The short answer - it's cheesy, but I'm not sure there's anything you can do besides get commiseration here.
I've never seen a case were the candidate or the prospective employer sign a confidentiality agreement that protects the candidates. And while it's a reasonable expectation, I can't see a case, unless they commit slander, where you have a legal right to get compensation for being harmed.
The most you can do here is avoid that particular business and tell others who trust you not to interview there.
The one thing I'll say is - I don't necessarily expect, when I interview, that the people I identify as references will be the ONLY people that are contacted when an employer is researching me. I've put my job history on my resume, and I expect that if the interviewers recognize the company and have personal connections to other people my company, that they may use their own personal connections to check my reputation. I will be clear when I interview if I'm being extremely discreet and clarify that I hope that the interviewing company will respect that - but there are no absolutes here.
I say this coming from a career history, where a certain part of my subject matter expertise is in a technology with a fairly tight community in my area. So... I do expect that if I'm talking to a fellow engineer/manager with similar expertise, that we will both have a certain number of shared connections.
Mileage varies significantly on this.