IpHONE APP DISPLAY

Is she supposed to have more than 3 screens? maybe she inadvertently hid a screen?

If you tap and hold on an open part of the Home Screen, apps will wiggle, tap the dots right above the dock. All the home screens will display in a tile form. If there are unchecked screens, then check em.. select Done

Organize the Home Screen and App Library on your iPhone - Apple Support

If she has more apps than displayed on the 3 pages, swipe all the way to the right to get to the App Library, hold your finger on the app icon then drag it out to the home screen.