Your mom shouldn't even be thinking about 'quitting' apps. She should just use her phone. Quitting apps is something only techy people obsess about. My mom is 67 and has had an iPad and an iPhone for almost a decade and barely knows how to get to the app switcher. Not having a way to remove recent apps from the task manager has not affected her in literally any way.
On many occasions, an app will not be working properly and she’ll call me. I have told her repeatedly to close the app, and she finally remembers to try that first before restarting, etc. It has worked well as that usually fixes any wonkiness. This would be a good feature for her.