So I backed up the essential parts of her system using Time Machine: documents (including her novel and decades of prior writing, published and unpublished), ten years of family photographs, a 41GB iTunes library, among other things, and then....backed it up again elsewhere, outside of Time Machine.
And just for good measure, once more time, to yet another drive. I wasn't going to be the (ex-)friend who lost her novel.
What I wanted to do was do the ol' "erase and install" that prior system software discs allowed you to do. But clicking around Snow Leopard left no obvious method for this. But instead of booting Snow Leopard while inside of Mac OS X, if you just directly reboot the computer off the disc itself (holding down the "c" button after the system chime, letting go when you see the Apple logo) you get a few more options. Once you've gone into the installer program, you'll see "Utilities" at the top, and if you select Disc Utility, you can see your hard drive. If you click on "erase" (like I did, with one hand over my eyes) you can wipe the drive clean with various security options, and then you can do a fresh install of Snow Leopard on your computer on a pristine hard drive.
Once you've done that, you can either transfer your files back in directly, or use the installer program's built-in migration utility to restore any or all files from Time Machine.
And now? The world awaits the Next Great American Novel, untragically unlost by yours truly.