You really need to hire an attorney if a civil lawsuit has been filed under the facts you describe above.
The above is general legal and business analysis. It is not "legal advise" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
I agree with the previously posted answers. However this is not a civil rights question as such as there appears to be no public agency or governmental body involved. You may wish to post your inquiry in another category.
Please see my general guide to defense of defamation in California.
This answer is not a substitute for legal advice and it does not create an attorney-client relationship. Seek the advice of a licensed attorney before taking any action that may affect your rights