Hello, this is emily.
This is NOT your Microsoft account password, but your current local account's password.
If your local account doesn't have a password, use this instruction to set up one:
Hold down Ctrl + Alt + Del, click on "Change password".
For the current password field, leave it blank.
Then set up a new password.
Once set, go back to what you were doing. When prompted for Windows password, use the one you just set up.
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Another alternative is to hold down Windows + R keys, type in netplwiz and press enter. Then click on Add. and see if it allows you to sign into her Microsoft account. Once signed in successfully, back to netplwiz page, double click on her user, go to membership tab and make it admin. Restart the computer and her user should be on the lower left.