As you likely already know, anything you do is fraught with potential negative effects on your business.
Leave the review up with no response and people think this may be true. Respond too forcefully, and people might think you are a bully to boot.
You seem - and this is to be expected - to want to make an emotional response. That would be ill-advised.
You have to make smart business decision, including how you respond to social media. In this day and age especially how you respond to social media.
My advice: Respond like a businessman.
Apologize for any misunderstanding but state your belief that no one would tell your patients (established or prospective) these things and that you and your staff, who have been in business for five years, strive to offer the highest quality services from before the first appt is made. Explain how Mitzi has 40 years of experience in the same location. State that you would appreciate the opportunity to earn Mr Wilson's business.
Bottom line: He probably doesn't want you and you probably don't want him but this is an opportunity to show that you are level-headed and care about people.
This is your opportunity to advertise that for free.
I would also advise that you pay extra attention to Mitzi's dealings with the public to make sure she is behaving appropriately unless you are already 100% sure that she would never say anything that could be misinterpreted or misunderstood in this way by any reasonable person.
Good people are very difficult to find for that type of position. It can be very demanding and takes a particular set of skills that seem more and more difficult to find and keep, so doing the above with some stealth and only if you have a doubt is in order.
Running a business is hard. Building a reputation is difficult. 5 years is long enough to build a pretty good base and those who know you and your staff will know the truth, but you want to continue to build so responding with finesse and grace will help in that endeavor.
To sum it up:
Divorce your emotions from your response and use this as an opportunity to grow your business.
FWIW, we've been through about 3 Mitzi's in less than 2 years, but each had issues that could not be overcome. I've been here three years and building our base has taken a long time and we still have a ways to go. Two things have stood in the way: Poor decision making by former "leaders" and poor decisions by staff (that was too often directly or indirectly caused or made worse by poor leadership decisions). We've had to delay expanding services so we can get the right pieces in the right places. That's how vital getting and keeping the right people is.
Good luck, keep a level head, be a businessman, and provide leadership and all will be well.