Not a legal requirement but I find it a bit odd that the manager ran the exit interview instead of HR. Normally HR likes to handle these, to get a second non-manager opinion on what just occurred and to make sure that the manager does not somehow put his/her foot (any further) into it. But there are no legal requirements on who runs the exit interview, on what was said, or even if an exit interview occurs. If Bob is the manager and he fires the last 5 employees he hired for cause, it is possible that HR will want to (and should want to) talk to the exiting employees and find out if there is something they need to know about.
Having said that, most terminations are legal, including the stupid ones. I worked at a place in the 1980s where the company president was very P.O. at the factory supervisors and told HR to fire one a week (chosen at random) until the group got their act together. HR was horrified but our outside attorney deemed the action legal.
There is no law that says the manager must be nice or actually know how to do their job. The very few illegal terminations involve an actual law being violated, like Title VII or ADA. The manager being a jerk or misguided in not recognizing how wonderful the employee is does not violate any actual laws. And the employee thinking that they are wonderful does not legally make it so.