On my Flex the setting is under 'gestures'. It will NEVER wake up in her purse except by the power button getting hit, same as any other phone. Mainly it uses a few extra battery % per day checking the touch layer. In my experience the Flex does not wake up in pockets even when facing my chest, leg or career. The sensor is pretty smart, better than M8 proneness to gesture wake I think. But I have not spent a week with an M8 in my shirt pocket to test for myself....
The other places most likely to check are Display and Lock Screen. Also you should explore and see if the comprehensive LG Backup suite is in your settings options.