I do not practice law in Florida however the rules of "defamation" are pretty much the same. Your question is a bit to oblique for me to give you a direct response. In general however you are not being defamed unless someone says untrue things about you TO OTHERS. Saying it to you is not defamation. In addition, these defaming statements must be believed by others and you must be damaged in some way.
Additionally, the workplace is not responsible for the actions of their employee when she is off duty and not at the place of employ. If she is using their establishment to make the calls that may change things but if she is standing there, using her cellphone then they are no more liable than if she were standing in a park.
Now to look at what is really going on, you are being harassed. You need to take the texts to the police and ask them to back off this person. Assuming there isn't further damage you probably do not have a civil case but this person can be arrested for harassing you by phone if she does not stop.
Involve the police in the investigation. Keep a record of all the problems. If you sustain damages record all of them.
You might find my Legal Guide helpful "How to Choose A Lawyer For You"
You might find my Legal Guide helpful " What Do I Tell My Lawyer"
You need a lawyer licensed in FL. Check with a lawyer in your locale to discuss more of the details. Have all the documents ready when you start phoning around to get counsel.
Good luck to you.
God bless. I am in Chicago, IL. I do not practice in Orlando.
NOTE: This answer is made available by the out-of-state lawyer for educational purposes only. By using or participating in this site you understand that there is no attorney client privilege between you and the attorney responding. This site should not be used as a substitute for competent legal advice from a licensed professional attorney that practices in FL in the subject practice discipline and with whom you have an atttorney client relationship along with all the privileges that relationship provides. The law changes frequently and varies from jurisdiction to jurisdiction. The information and materials provided are general in nature, and may not apply to a specific factual or legal circumstance described in the question.
Sign up to receive a 3-part series of useful information and advice about personal injury law.