Try clearing cache.
-Turn the phone off
-hold volume down + power until the screen turns on
-release power but continue holding volume down until you see fast boot at the top of the screen
-press volume down 7 times until it says android recovery
-press volume up
-at the triangle screen press volume up + volume down simoutaneously
-navigate with volume buttons and power to select, wipe cache, reboot.
If that doesn't work, are you willing to factory reset? It will erase ALL data from the phone.
Has he installed anything recently that could be the culprit?
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