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.