The problem here is not the k word or n.....r,or h.....y word,its the reasoning behind deciding to use the word.
H....ey was developed by black people to counteract the n....r word,so it follows that it will be less dramatic been used by black to white,but unacceptable.
Now when a black calls a black by the n word,the reasoning is different from when a white calls a black n or k word,at least in most cases.
The Captain here had a wide choice of non-racial abusive words like "prick,as..le,id..ot,dimwit e.t.c,so his choice was well thought out,or so he thought.
Anyways if your co policy forbids it,then abide by it,but firing him,a bit dramatic in my own opinion,since it was not to his face.
Its a nightmare this hotmic thingy.