Can you force applications to open in full screen? Is there a way, like an app or a system preference, that will open applications in full screen, all the time. By full screen I mean taking up all the screen real estate. I know it doesn't match apple user interface guidelines but I have a friend for whom it would make life easier.
Currently, no. Through the Macs history (as you pointed out with the HIG), Macs have had a 'zoom' feature versus a maximize one like Windows. Many times this will make the window full screen, but not always. 

With Mac OS 10.7 due out this summer, having a full screen application is one of the new features, where the application will take up everything on the screen. Since 10.7 is still under NDA, not sure if there will be a way to have the application always launch full screen or not.