Wow! You got me on that one
The only thing I can think of is: does your friend have other animals? or maybe there used to be other animals that the cat can smell? Maybe even a certain noise close by he can hear tha t bothers him? I know one of my cats does not like the sound of the gas weed eater (even if it's the neighbor), and he runs to hide behind the stereo

Other than that, I don't know