Re: "Settings for someone who doesn't know how to read? Is there any way that she can turn on the screen reader feature whenever she wants to? instead of it being turned on all the time?"
Good to know this elderly woman has you to help her stay connected!
If it is the "Speak Screen" feature that is in use for her.
Once it is turned on (in Settings > Accessibility > Spoken Content > Speak Screen)
The method below may be useful.
To start listening to the reader: Swipe down from the top edge of the screen with two fingers
If you want to pause: Tap the screen with two fingers.
To stop altogether: swipe up from the bottom of the screen with two fingers
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Another method is to use Siri: Say something like: “Speak screen.”Learn how to use Siri.
Apple Support's online article has more details, if needed:
Hear iPhone speak the screen, selected text, and typing feedback - Apple Support