The PowePC processor was abandoned in 2006 so it's hard today to find new software that supports it. PPC compatibility is a function of the software developers, not where you buy. Example, TurboTax still runs on G4 computers. Most, however, does not.
You can still get copies of Office 2008 here:
http://eshop.macsales.com/item/Microsoft/GZA00006/
It was the last version that ran on PPC. I have it on a PPC G4 1.25Ghz tower and a 1Ghz G4 PowerBook and it runs fine.
There are free suites that do what Office can. There is a PPC version of NeoOffice on this page:
http://www.neooffice.org/neojava/en/download.php
You can get PPC version of NeoOffice here, also free:
http://www.libreoffice.org/download/
If you post what kind of software you feel Dad needs, we can come up with some other suggestions. The software is out there, but takes some looking.
A Mac User Group may be your best resource for older software. This page will help you find one close:
http://appleusergroupresources.com/?page_id=394