Unless Apple started injecting payloads there's basically no plausible way to get her infected. She doesn't even "browse the net" for the most part, doesn't click on links, doesn't give a f.
There are safe habits. AV companies would like to have you thinking you're always about to have your nix based system rooted, but this is damn unlikely for most people not using dodgy sites. I fancy my chances to get struck by lightning above her chances of having her system compromised, and I don't get out of my house scared.