Since a MacBook Pro seems to be a viable option (budget wise), I'll go ahead and recommend a 2017 12.9" iPad Pro with a Brydge keyboard.

Why?

• The thicker bezels make it easier to hold onto it without (occasionally) triggering unintended inputs.
• iOS is super forgiving and tech support is easier on you.
• Once set up, backups don't require any user attention.
• The large screen is nice to read on.
• The large screen and the forgiving nature of iOS'es touch input is really nice for people with decreased dexterity.
• ...

Unless there is a specific need for some software that is unavailable for iOS, I'd strongly recommend going the iPad route. It's what I did with my parents and it's made their (and my life) considerably easier. There are just so many fewer things that can (and eventually will) go wrong...