It's not fair of the boss to be so down on a new employee.
fair，unfair 跟 kind 用法一样，属于描写人的品质、身份、特征的形容词，才可用于该结构：
It is kind of sb. to do sth.
It is not fair （=unfair）of sb. to do sth.

如果变化句式，应这样：

Sb. is kind to do sth.
Sb. is not fair （=unfair） to do sth.

The boss is not fair to be so down on a new employee.

be down （副词）on（介词）：对......不满。

网友的转换不正确。