I wouldnt even worry about that(or mention it) ,the fact is someones used something thats reacted with the paint,they knew it,and wheeled it out to you knowing full well it was buggered.Theres no excuse for that ,and frankly i wouldnt think twice about someones job,if thats the kind of level of service they think is acceptable then they shouldnt be in the job anyway,someone more deserving should have it.They didnt give a fuck about you or your car,so no sentiment in return needed there.Really hope you get it sorted mate,i cant believe they thought theyd get away with it.They should have pointed it out in the first place.