I would kindly but strongly recommend you retain a lawyer ASAP to forward this person a "cease and desist" letter, and hopefully "scare" them into ceasing this conduct. If the allegations are untrue, you may have a cause of action for Defamation, so please ensure you hire an attorney to shut this person down. I am sorry you are going through this, and I wish you all the best of luck. Warmest regards, Matt Catchick.
As my colleague suggests, your first step is to retain an attorney. If the person does not back down after the letter or if they do make a false statement which harms you, your attorney will already be familiar with the case and can sue. Likely, can can also reassure your employer, if it comes to that.
We do not have a client/attorney relationship until you make an appointment, we discuss your case face to face, I accept a retainer, and we explictly agree to enter into representation.
I agree with my brother counsel, you need to hire an attorney and if this is indeed blackmail, you are being asked to pay money or take some action in order to prevent this allegation from being revealed it is also the crime of extortion and it may be that the attorney you consult will recommend you file a claim with the authorities.
If you know this is coming you may wish to discuss the false allegations with your employer in order to minimize their effect.
To the PROSPECTIVE client, please call myself or another attorney for your choice with more detaiils and an appointment. My PRELIMINARY answer to your question(s) is for general purposes and based upon what little information you have conveyed. It is based on such limited information that the general answer should never be relied as a reason for your action or inaction. My response does NOT establish an attorney-client relationship and such may only be established by mutual agreement, and the signing of a written retainer agreement, which will generally require payment for our services, as this is what we do for a living and, just like you, we must get paid for our work.. .