Mail app on iPad mini (2012, running iOS9) not working

Sorry to hear your mother is having problems with her iPad. I want to start off by saying that iOS 9 is no longer supported and no longer receives updates, so if this is a bug with the operating system, it will not be fixed.

First, make sure her iPad is on the latest version. Open Settings, tap General and Software Update. Check for available updates. The latest version for your iPad mini is iOS 9.3.5.

If there are no new updates available, start off by simply restarting the device. You can do this by holding down the power button, sliding your finger on the slider and turning the device back on. Once you're back on the home screen, trying opening Mail.

Note: You cannot delete or reinstall any built in apps (including Mail) with iOS 9. This feature was introduced with iOS 10 and improved with iOS 11.

If this hasn't resolved the issue, open Settings, tap Mail, Contacts and Calendars and then choose the option to remove the existing email account. Add the email account again by tapping Add Account and enter your mother's account information.

One final thing I can suggest is to use the webmail version of her email service. Email providers like Outlook/Hotmail, Gmail and Yahoo offer a web interface. Open Safari, search for the email provider's webmail (eg. "outlook.com login"), sign in on the respective page and you should be able to manage your email on that page. If you want to bookmark this to the home screen, tap the Share icon in the Safari toolbar, select Add To Homescreen, then tap Add.