just plug your memory stick in the computer and then goto my computer, then right click on the flash drive and click on format,
it could take a while though, depending on what size stick you have, i have a 2GB and an 8GB and i had to reformat the 2GB once and it took about and hour
sometimes that method does not work. He will need to go to control panel - computer management - disc management - and reformat the drive from there. After which he will need to assign the drive a new letter.