Make it keyboard Navigable (Score 1)

I have gone through this process with my wife and a couple of friends, all much younger than your grandfather and all of whom were going from typewriter era to gui. I can attest that the mouse based interface is one of the hardest things to learn if you haven't grown up with it. That being said I think Mac would be uniquely difficult since it is so maximally graphical. What worked well for my wife was a DOS setup with a text mode, keybord driven menu. It has only about 4 entries for the programs she uses most, and the only troubles have come when she has to use a "modern" gui based program.

I imagine you could do something like this in either linux or win 9x with a little clever scripting. Finally, based on my experience the earlier advice of using text based apps (pine, lynx etc.) seems good.

From one of our Veterinarian friends: "If I were meant to play with a mouse I would have been a cat!"