there isnt a proximity sensor. its software one using mic and top loud speaker to mimic.it is just a suggestion u may want to try the proximity sensor reset/fix app by tech tool
see if this help?
or what casing are you using bro?
what kind of screen protector ? ,
or you might want to clean the top of the phone where the sensor is ?
It only works when the phone is vertical iirc, maybe his phone hold slanted and it doesnt activate.