My Mama use to have a lot of saying... 2 come to mind here..

#1...NEVER argue with idiots or drunks!

#1... It is OK to be stupid! Just keep you mouth shut so the rest of the world won't find out..

Both seem to fit that coworker.

(I listen to my Mama, even today. I will NOT aruge with idiots or drunks.)

... and HE is just a IDIOT!