The employee can still sue. There’s a reason why others say to keep documentation of everything in situations like this. While they don’t have to tell you why you’re fired, if you sue, they still have to provide adaquate reasoning. Can’t really say "I just don’t like the guy anymore’ and have that be sufficient.

There’s no way for us to know who’s really in the right here since we don’t know what the specifics of his employment agreement are. We can just agree that the employer is wrong, and stupid. Why piss off employees that actually do the work?