Hello
Unless the account is a local user, your other administrator has no way to reset PIN or password if an account is linked to Microsoft account.
You said it requires a Windows Hello PIN to be set up, may I ask for some clarification? Does it ask you to set up PIN but doesnt offer you the steps there to set it up? Were you able to log into that user before?
To make sure it is indeed properly linked to a Microsoft account, sign into your other admin, then hold down Win+R keys, type in netplwiz and press enter. When it is launched, does it list the other user's email address?