Dead spot on screen

If it only happens within certain apps, the fault will be in the app itself.

Ask the user to uninstall the app, then do a soft reboot & reinstall it.

 

If the area is dead, no matter which app is open -

Ask the user if there are any signs of physical damage - cracked screen, bleeding or lines (cracks may be very faint).

If there is physical damage or the phone is out of warranty, offer to get a quote for repair.

 

If there is no physical damage, ask the user to perform a soft reboot.

If that doesn't work, perform a full factory wipe & restore.

If the problem persists, arrange a warranty exchange LINK  or get a quote for repair.