I screwed my usb flash drive too, idk how bad it was to your friend, but i had everything placed in folders, so writing in a path to the folder to recover it is easy, opening the properties of the folder then can help to unhide it. However if it caught something serious try formatting however all data will be lost. So basicly what is the problem? can't see files? the usb doesn't load up? or is it corrupt with a virus?
Ok this will sound rally wierd, but trust me it can actually work, just throw it in the freezer for 30minutes or so, then try it. by lowering its temp it will have an easier time making needed connections etc