If you have experience of asking for an "App Password" . Please update the client to confirm that the 2FA needs to be turned on in order to generate the " App Password". Sample login as per below .
https://portal.smartertools.com/kb/a3625/setting-up-two-step-authentication-2fa-mfa-etc_.aspx
An app password is a 16-digit passcode that gives a less secure app or device permission to access your Google Account. App passwords can only be used with accounts that have 2-Step Verification turned on.
NOTE
Outlook Classic is recommended instead of Outlook New for the above setup due to compatibility issue.