For non tech users, iOS is pretty safe.
Ipad + iPhone and you're good to go. Leave the notebooks and PCs in the 80s where they belong.
All she needs is email and web browsing. I'm pretty sure as far as security threats are concerned she is vastly better of with Linux. The usual exploits targeted at end users simply won't work.
What makes you say "Avoid Linux"?
No security is perfect, a small handful of bugs over the last so-many years is about as good as any platform can hope for.
If you're a CIA agent or run a bitcoin exchange it might not be good enough for you, but for most people it's a pretty good set of options.