Very interesting, but this isn't at all about suing for slander, it's about the deplorable actions taken to attempt to silence the consumer that was clearly mistreated in this case. Opportunity Lost! I'd bet $100 that this is going to be very, very expensive for the Dr. that tried to bully a consumer into removing an accurate representation of their experience. The irony is that their effort to silence the consumer, presumably to protect their reputation, netted quite a different result.
Quoting Blake Z again from an earlier thread, but the best Review strategy is to "give excellent service, really excellent service." The negative review should have been the wake up call that alerted the Dr. to several broken and unfriendly processes at their practice. Reach out, make it right and fix your process to prevent clerical errors in the future.
This is a great example of one of the "real" value propositions of seeking reviews for a business. If a customer alerts you to an unfriendly or broken process, take that feedback for what it is and look honestly at the process from a consumer's perspective. They are likely not the only customer that felt that way. Reviews aren't solely a marketing tool, they are also a great insight into your procedures and processes from a consumer perspective.
Giving "Excellent Service" to earn a positive review and truly, honestly evaluating the content of a negative review shouldn't be as painful as, say, pulling teeth...