Hey there! Nice to meet you!
I'm sorry you and your mother are experiencing this.
Since she does not have access to the verification method (the recovery email address) for a password reset, I'm afraid the only way to recover the account is through the account recovery web form: https://account.live.com/acsr
Important: If she has an active 2FA, ACSR will inform you that your recovery request will be ignored. If you only receive a message that the recovery request is either successful or indicates insufficient information, this means that 2FA is disabled.
You'll need to keep trying until you're verified by Microsoft, and you can do this up to 2 times every 24 hours.
If your Microsoft account recovery request was not granted, please review this link for tips on improving your chances of a successful recovery: https://support.microsoft.com/office/dc56c3e3-dff0-4ed5-8edb-60290eea6b53
Please be advised that Microsoft Support will not be able to assist with manual recovery and escalation for this case, as stated in this article, that they are not permitted to send password resets, access, or even change any details on the account:
Thank you for your understanding.
Best regards,
Darhyl