If your Air has 2 GB of RAM or more (4 GB or more HIGHLY recommended), you could try Parallels Desktop or VMWare Fusion. It looks like there are trial versions of both. You'll need a copy of Windows that isn't legally or technically bound to your PC (whether you can migrate your current...
He can't install Windows directly on an external drive (due to Microsoft restrictions on Windows booting, and Apple restrictions on Boot Camp).
On the other hand, if he installs a virtualization program (Parallels Desktop or VMware Fusion), he probably can put one or more Windows virtual...
One-stop shopping for identity thieves?
I hope that your master password is long, complex (upper and lower case + numbers + special symbols), and hard for anyone else (especially thieves) to guess.