Disable all iOS notifications
Until recently the latest version of iOS available for the iPhone 5 was iOS 10.3.3. 

iOS 10.3.4 and 9.3.6 were updates for older devices (released at the same time as iOS 12.4) to fix a problem that may cause problems with GPS location performance and/or lead to the system date and time being incorrect. It's only because of the nature of the bug they were fixing that Apple released these updates years after the last update for these older versions of iOS.

So, the reason for my explaining all this is that it'd be best to just update the iPhone 5 you gave your grandmother and then the likelihood of another system notification is extremely unlikely (iOS 10.3.3 was release about two years before this one).

For more information on this update, refer to: Update your iPhone or iPad software to avoid issues with location, date, and time on Apple's website.