Hello,
Being signed into Windows does not mean the online Microsoft account is still accessible. Windows is currently using previously cached credentials or a PIN. That Windows session cannot be used to bypass Microsoft’s account-verification process unfortunately.
Unfortunately here we are not Microsoft support, but community forum helper. So there's no way anyone here can review the account or recover the account for you. The best we can do here is to provide you with information (whether it is good news or bad news) and point you to the right direction if the option exists.
Have you already tried the Microsoft’s Sign-in Helper, which identifies the appropriate recovery process for locked or compromised accounts. https://support.microsoft.com/en-us/accounts-billing/manage/how-to-recover-a-hacked-or-compromised-microsoft-account
If directed to the account recovery form, complete it from your father’s usual computer, at his usual location and internet connection. Include as much accurate information as possible, such as:
Previous passwords
Original name, birth date and address used when the account was created
Exact Outlook email subject lines and contacts
Skype or Xbox information, if applicable
Microsoft purchases or subscription details
The most recent emails he sent or received on that account (I heard this one usually increased chance of approval significantly)
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I also want to point out, Microsoft support cannot manually replace the security information, reveal the correct answers, or send a password-reset link. If two-step verification was enabled and none of the registered verification methods or recovery code are available, Microsoft states that the account cannot be recovered.
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Since Windows still opens, back up all important local files now and create a separate local administrator account before signing out, resetting Windows or removing the Microsoft account. This will prevent losing access to the computer if the cached sign-in later stops working.