If he has an older mac, then it'll be tricky to get a USB drive to boot, in which case you'll be better off burning a CD/DVD. In my experience on my mid-2007 iMac, I couldn't get a USB drive to boot unless I used the
PLOP Boot Manager.
1) Put the ISO on your USB drive using the pendrivelinux installer as suggested above, since I think it gives you persistent storage.
2) Burn the PLOP Boot Manager to a CD.
3) Insert your USB drive.
4) Boot the CD by holding "C" when you start your computer, or by holding alt/option at startup, followed by selecting the "Windows" CD.
5) Select USB in PLOP's menu. (I don't know if I just had a bad burn, but the disc completely stops spinning after a bit and then freezes up, so be quick when PLOP loads up.)
I don't know what generation macs became friendly with booting USB drives, so as I said, you might be better off burning a DVD. And maybe give them a blank USB drive to save their stuff on.
EDIT: Of course, I forgot to give a link.