Maybe like me he has developed chronic RSI from to much game time using a mouse, its quite common. Yes using a mouse and keyboard MAY be better for FPS but the controller is great to give the RSI a break to heal.

At the end of the day you cant really say that a mouse+keyboard set up is better as its all about adapting your reflexes so if you play long enough on one or the other you will be able to play perfectly fine, it doesnt matter.

Only real advantage with keyboard are all the extra buttons.