Just in case you haven't seen any of the updates elsewhere, there is an Accessibility setting to turn off multi-tasking/split-screen that would most likely help

Just in case you haven't seen any of the updates elsewhere, there is an Accessibility setting to turn off multi-tasking/split-screen that would most likely help your grandmother: https://support.apple.com/en-us/HT207582 https://support.apple.com/en-us/HT207582