Wouldn't the easiest solution be to just select "Find File" in the finder, search for ".mp3", and then drag everything that comes up to the trash? That should be all it takes if he only has to do it once--not quite as smooth as a double-clickable script, but not that big a deal.
If he needs to automate it as a shutdown task or something, then I can't help, but I'm sure some Applescript guru will.