User with 2FA authentication code not received, is usually connected to some problem with app, or Mobile Phone that had to undergo a hard reset, and lost all data.
Administrator can try to proceed with 2FA reset for the affected user, under: Setup > User/Roles > Two-Factor Reset Tool, but that will generate following error:
You cannot reset the 2FA settings for this user. Either the email address entered is invalid, or you are not allowed to initiate a reset of 2FA settings for this user. You can only initiate a reset of the 2FA settings if you are an Administrator in all accounts where this user has non-customer access.