This is something that I believe almost every barn owner will go through during their career in the horse industry. I definitely have had my taste of angry ex-clients and with social media it is even easier for an ex-boarder to run your name through the mud.
The second thing I would like for you to remember is that you have a very supportive team and group of clients at your barn that stay because they love the way you run your barn. They trust you and recognize great care when they see it. They will be your strongest supporters and even defend you if they hear negative talk out on the streets.
Now if this person is saying terrible things in emails or social media and it is out there for the world to see, then I would advise you to talk with your attorney and find out what steps need to be taken to stop this. If he sees this as clearly a case of defamation of character then he will advise you on what to do next. Most of the time the person doing the talking will back right down and "quiet down" if they know you have sought professional help from a lawyer.
It is never easy when a disgruntled boarder starts talking and making life stressful for the barn owner or manager but it happens. I would like to encourage you to take the high road and don't respond back at all. Any talk you give back will only fuel the fire. Just keep doing what you do best and give great care to the horses at your barn and people will see what the truth is. Honesty, integrity and great care always win.
Thank you for the great question,