Normally this happens after a crash while updating firmware.
Try booting the phone in safe mode and recovery mode.
If you can ask the customer to longing in google account and if he's OK with resetting the phone
Press and then hold the Home and Volume UP keys, then press and hold the Power key.
When the Samsung Galaxy S7 shows on the screen, release the Power key but continue holding the Home and Volume Up keys.
When the Android logo shows, you may release both keys and leave the phone be for about 30 to 60 seconds.
NOTE: The “Installing system update” message may show on the screen for several seconds before displaying the Android system recovery menu. This is just the first phase of the entire process.
Using the Volume Down key, navigate through the options and highlight ‘wipe data / factory reset.’
Once highlighted, you may press the Power key to select it.
Now highlight the option ‘Yes — delete all user data’ using the Volume Down key and press the Power button to select it.
Wait until the phone is finished doing the Master Reset. Once completed, highlight ----Rebooting system now then press the Power key. The phone will now reboot .........bit longer than usual.