Of course they can take legal action. Anyone can sue for what they believe to be a breach of contract. Just because they can sue does not mean they will win or are correct. I doubt that they can actively prevent your father from leaving Florida simply because of their contract. Perhaps the contract did not allow taking the rental car out of the state?
The question here is what kind of legal action do they mean. If they mean filing a civil lawsuit then no one can prevent that from happening and it will have to be dealt with in court. If they mean calling the police and having criminal charges then your father should absolutely not speak to the police and must invoke his right to remain silent. If this is a dispute over a contract and the car was returned then it will likely be a civil matter.
This is not to be considered legal advice nor does an attorney-client relationship exist.