Can a boss call a worker a fool?
A boss has no more right to call a worker a fool than anybody else does.
Under ordinary circumstances, it is forbidden to call somebody a fool, because this is liable to cause him distress, and falls under the category of ona’as devarim. Only under exteme circumstances, where strong rebuke is required (for instance, where somebody is repeatedly sinful), might it be permitted to use such terms.
Even if a worker makes mistakes, a boss should try to refrain from insults, which as a general rule will not benefit the worker, nor himself.