Hello Joyo. He's likely obscuring the proximity sensor near the top of the screen, about the fist half inch or so of the screen has embedded sensors to help keep the screen black when it's near the face during calls to avoid accidental icon activation on the screen.

Maybe it's dirty and keeping the sensor "thinking" the phone is still near his face. Have him clean that area with a soft cloth or bit of cotton ball. It has to be done periodically.

Maybe it's the position of his hand as he holds the device after a call; blocking the sensors again, making it behave as if the call is still live.

 

If you are rooted and locking up try way bumping up your CPU frequency. That generally worked for me but killed the battery.