In 25 years of practice I have never experienced this. Finally decided to fire a difficult client (drama, late on compensation, non-payment for late cancellations and verbally abusive)and now she is threatening to ruin my career by writing about me on the internet among other things. Does anyone have any experience with this or advice????
If it comes down to it you can or threaten to sue her for slander of your name.
It would cost you a lawyer to right up the letter but I would suggest doing it before she starts writing negative reviews about you. Once people see the negative you more than likely already lost them.