I would still guess it has to do with the battery's age and the battery temp as it was charging while your mother was actively using it. This is the kind of thing that Apple was trying to prevent with its now infamous built-in throttling as their phones aged, thus causing the phone to slow down. The purpose was to decrease CPU usage and therefore stress on the battery as it aged, because if there was enough power demand that the battery was unable to meet, the phone could suddenly shut off.