another legal question

a friend of mine bought a car stereo at a stereo shop a while back and had it installed for about $40.00. the radio had a 1 yr. warranty on it. then it went out. they replaced it. they said that the problem was with the ohms of the speakers. well it went out agin, this time because of shoty work. the power wire and speaker wires were all hangin out behind the brake pedal and caused a short. now they want charge him for the repairs. what legal rights does he have?