Your client/friend essentially has three options.
- Install Windows to a partition via Bootcamp (or a similar solution).
- Install Windows via a virtual machine.
- Use a Windows Emulator.
Now, since you have said that installing Windows via a VM would be a bit beyond them, that pretty much eliminates options 1. and 2. That leaves your friend with needing to use a Windows Emulator.
You could have your client/friend use...
- Wine. This would probably be the most robust emulator, but the most complicated to set up. As has been pointed out, you may have difficulty finding a pre-compiled Wine build for Mac OSX, so feel free to use one of these. As per this page, there aren't many pre-compiled builds (and no official) for Mac OSX out there.
- Winebottler. It's BETA, it uses WINE, but it is going to be a little friendlier. Of course, this doesn't mean it won't be buggy.
- purchase Crossover. It's a commercial product, so it is going to be polished, easy to setup and use, and user friendly. That's why it's not free.
Of course, you could find your own Windows Emulator, but this is the direction you are going to have to go in, regardless.