Your employer does not have any control over what someone says on social media, nor should they get involved in interpersonal disputes between employees. They can reasonably expect all employees to be civil while at work and not impede the company's mission but they are not your parents nor a referree. There is a slight exception if it is work related and has the potential to impact the company directly. Then, I would bring it to the attention of the powers that be.
"Jojo kicks puppies and clubs baby seals"- you are on your own
"Jojo, an accountant for XYZ Corp., has been embezzling funds and falsifying reports to investors"- report
Either way you have zero case against the employer.