how did the revenge turn out?
if you just want to scare him with that screwing the gf on the bonnet, get some condoms, those clear/grey glue that we all used in school - dip the condom in water, put some of the glue in there, and then place it on top of the bonnet - then do the same and place it on the roof of his car - while ur at it, smear some of the glue on the door handles too
then get him to read the thread about how to get rid of "cum stains"!!!
the guy will be scared that his gf has been getting around, and (2) he wouldnt want to touch the car.....