That usually happens when...
1. The battery is old/damaged, and needs replacing.
2. There is an app on the device that is using a lot of battery, even when you aren't using that app. The only way to prevent that is to remove the app that is causing it.
3. Whatever she was doing on the tablet (during the 14 minutes) uses a lot of power. Which is completely possible.
I would seriously consider it being the battery though. Since the tablet sat for a year, and was probably used for a while prior to that, the battery may just be done for.