Calling boss ‘Scrooge’ online: Protected?

Q. An employee brought to the attention of his supervisor that a co-worker had posted a comment on social media saying that her supervisor is Scrooge, that the supervisor is probably planning to fire a bunch of people right before the holidays, and that everyone should complain about her unfair behavior so that the supervisor is the one who will get fired. The company has a social media policy that prohibits making disparaging comments about it or its employees. Can the company discipline the posting co-worker for these comments?