[ATDrake]

There's never been a version of Kindle for Mac which supported PowerPC. Similarly, B&N's NookStudy/Nook for Mac has always been Intel-only (though the older B&N eReader did support PPC but is now no longer available from them; but you can still open B&N PDBs using the even older app from eReader.com).

As mentioned, the Sony Reader Library app will do ADE DRM-ed ePubs and she can use that to read e-books from the library, Sony, and Kobo as well as the DRM-free stuff.

The alternative for Kindle use on a PPC Mac is to have a hardware Kindle and strip using the tools we don't talk about on MR, at which point she can read the liberated Mobi books on the laptop with an older version of Calibre.

Hope this helps, and welcome to MobileRead!