I'm sorry to hear about your loss. So if it is a Microsoft account there is something you can do:
When in the Command Prompt as admin go to:
This should output all the users on the account. If you don't see anything that matches, it may have been created with a Microsoft account as you suspect.
When you sign in to Windows 10/11 using a Microsoft account, Windows creates a local account on the device, but the credentials for the local account are not stored on the computer. Instead, the local account is linked to your Microsoft account and uses your Microsoft account credentials to sign in.
When you sign in to Windows 10/11 using your Microsoft account, Windows sends a request to the Microsoft servers to verify your credentials, and if they are valid, you are signed in to the device. Your Microsoft account information, including your settings, files, and preferences, is then synced to the device.
The local account on the device is used to store your user profile, including your files, settings, and preferences, but the credentials for the local account are not stored on the device.
You have two options Reset the password (you may need access to the phone + e-mail)
To reset a Microsoft account password, you can follow these steps:
Go to the Microsoft account password reset page: Reset your password
Enter the Microsoft account email address or phone number associated with the account.
Follow the instructions to verify your identity, which may include answering security questions, receiving a security code via email or text message, or confirming information about the account.
Create a new password and confirm it.
Sign in to Windows 10 using the new password.
Note: If you are having trouble resetting your Microsoft account password, you may need to contact Microsoft support for assistance. They may ask you to provide additional information or documentation to verify your identity. The other option:
If you are an executor or administrator of the deceased person's estate, you can request access to the account by submitting a request through the Microsoft account recovery page: Recover your account
You will need to provide information about the account and proof of your relationship to the deceased person, such as a copy of the will or a death certificate.
In this specific case, you may need to contact Microsoft support. In the US: 1-800-MICROSOFT (1-800-642-7676)
Also for reference: Accessing a deceased family members Microsoft account