What was her last computer? Is she looking for a desktop or laptop? What kinds of things does she do on it? Does she still speak? When PALS can no longer operate a keyboard, options include dictation/voice control, a head mouse if they can move their neck, a tongue/chin mouse if they can't, eye gaze if they don't have any muscles that can operate a switch. iPads and iPhones can also be operated similarly. The other thing to consider is how close she is to needing a wheelchair and what capabilities she will need outside the home.
As for Mac vs. Windows, you will find some of each in terms of users here. Either can be configured according to her preferences, and supports each option I listed, through a switch interface such as Tecla, built-in accessibility settings, or a USB port that an alternative mouse plugs into. Whatever screen(s) she reads on, since ALS affects head movement, she may need a larger screen than she has had before, or a screen raised or mounted to a different height.
Best,
Laurie