your mate your would have to pulg his car into the mains over night to charge it back up again. taking the fact car alternators are 30% efficient at best (whole loop here not just one part), he would need at least 100hp of his cars engine output to drive the system.
If it were on full power, it enough the smash the windows, blow the doors off and melt most of the car. If a person was in the car with 28kW of "real" audio power, (not some crappy PMPO or other crap), they would die, 100% sure here, they would die.
Not only would your ear drums burst, your eyeballs would heat up, probably burst, your heart would have real difficulty in beating your lungs would collapse, and if that was not enough the temperature of the air would rise by about 200 degrees centregrade per second (assuming no loss to the outside).
Is your mate still alive? if he is I very much doubt he has a "real" 28kW sound syste in his car. If he does it is probably only ever run on full power with all the doors open and from about 100 yards away.