I'd recommend a nice, recent model iMac. Doesn't have to be especially powerful or huge.
The All-in-one concept should work out nicely for her.
Most 80's and older peeps don't have the best eyesight so a plain wallpaper and larger fonts might be a good idea. Think Contrast.
She may also find that a cheap Maccally keyboard is easier to read and use than the stylish aluminum keyboard. Contrast again. Larger letters too if you can find them.
For mice, disable any side buttons or extra features. Make sure Hot Corners is turned Off too. Nothing puts off older people faster than accidentally navigating to a previous page, popping up the Dashboard or activating Exposé just by gripping the "wrong" part of the mouse or mousing a corner. Slow down the mouse double-click and maybe the tracking and scrolling too as needed.
Keep the Dock items limited to just what she wants to do. Get rid of the iLife icons and whatnot to keep things simple.
Just got an 88YO Doctors PC all set before he shuffled off.... 😱 At least he had a great sense of humor about it. He would have been FAR better off with a Mac, particularly an iMac.
Have Fun,
Keri