I'm guessing that she lets it run almost completely out before she charges it. That's one of the fastest ways to ruin a lithium battery. You probably have to replace the battery.
Then install
GSam Battery Monitor, go to the 3 dots (lower right corner)/Preferences, scroll down to Alarms. Set the Battery above alarm for 81%. Set the Battery below alarm to 39%. Choose a very annoying notification tone* for both of them, so she plugs and unplugs the charger when she should. (Charging above 80% isn't too healthy for the battery either, but it's not as bad as getting close to 0.)
(*If you don't have anything annoying enough, voice record a can about 1/4 full of paper clips, thumbtacks,m etc., being shaken for about 10 seconds, very close to the mic. Then use
Ringtone Maker to raise the volume until it distorts. Put that file in a folder named Notifications [case counts] and you'll have a very annoying alarm.)