Is it possible that the phone is accidentally being turned on while in her pocket or purse, and then the microphone on the Google search bar widget is being accidentally pressed?
Any phantom touches? Maybe the screen near the microphone is falsely registering a touch.
Sorry, I meant do you ever notice other unintended touches during your routine usage? Phantom touches don't need anything nearby to register -- it might be a problem with the screen, or perhaps a screen protector.
What if you clear the Google app's cache and data?
Try wiping the cache partition first, if you haven't already: https://motorola-mobility-en-in.cus...prod_answer_detail/a_id/106533/p/30,6720,9392
It just seems like something is wrong. Send some feedback to Google via the Feedback option in the Google app's settings. Also send feedback to Lenovo, if there's an option to do that in the phone's system settings.