I dont know how good of friends you are with this guy, but if someone fucked with my car there'd be hell to pay, plus id tell them to lose my number and go fuck themselves.
Just doesn't seem like the smartest idea to mess with anothers car, but it all depends if he would find it funny or if hes the type that would freak out and go over board.
Also, the horn wired to the turn signal was already done on TopGear, but damn was it funny.