Good and bad are opposite sides of the same coin! It comes down to taste and system matching!
A Naim system does certain things very well and doesn't do other things. So it depends on what your friend finds pleasing! A Naim system has propulsive bass, a warm midrange, and is quite dynamic.
An ARC system depending on the setup will throw a much better sound stage, and will have depth to the image. However the bass and dynamics may not be as force full.
So it comes down to system matching and preference it is possible that the ARC system was not well matched to the room or the speakers and the Naim was, who knows, but the point is moot, people will like what they like and sometimes a different sound is what that person was craving.
"There is nothing either good or bad, but thinking makes it so" - Shakespeare