Though, three week is a long time, to bring a racket back and say he did not like the feel. I get rackets back in a two weeks to be restrung. I'm with Steve on this, make him pay for the string and comp the labor, if he's going to be an ahole about it.
It's like going to the mechanic to get an oil changed, then comeing back 3000 miles later and saying you wanted synthetic oil instead of regular oil.
Machines: Gamma 6004 2-point w/ Wise 2086 & Babolat Sensor Dual