What can I do if I forgot my Apple ID? Is there a way I can recover it?

Tell him to check his web browser autofill entries, the Keychain, and other records, or use Apple's iForgot service, or contact their Account Security team, or the unit's original owner to click here and submit an unlock request.

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The Apple ID is almost always the email address of the user. The issue might really be a forgotten password for the Apple ID, since the lock goes into effect when the wrong password is entered too many times.

If you forgot your Apple ID password - Apple Support