There is a difference between threatening and nasty. Communcations to you alone only become a crime if physical harm is threatened. Where things are made public, then defemation might apply. To find out, you should go over the details with an attorney in your locale. DO NOT respond or make any online or telephone communications. "Bashing" is not a crime or a tort, but defamation is. See can attorney, who can analyze the case. Make copies now of the posts, in case they are deleted. You can control your Facebook site, and close it in to only friends.
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Sage advice from Barry on how to make sure you are preserving the evidence and getting accurate advice from counsel in your area.
I would add that you should understand that the threat of physical harm assessment can be very complicated, including issues of imminent threat of harm, known identity of person 'bashing'.
Defamation is much more subject to issues of opinion versus assertion of truth. Though, in 'bashing', most people seem to assert something false as truth, in my experience, yet they think they are voicing their opinion, thereby subjecting themselves to liability.
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You possibly have 2 different ways to combat this situation. First, it will be difficult to prove threats but she can argue her language was only inflammatory and not indicative of her behavior. Police have difficulties preventing/rooting out crime before it happens but are much more reactionary. The better tactic would be to consult an attorney about pursuing a defamation/libel claim against her because her behavior seems to violate these rules.
Answers to this question are for general purposes only and do not establish an attorney-client relationship or constitute legal advice