Can you tell us more about your environment? Are passwords required to control access to certain parts of workbooks? Is the user your boss? That's the only reason I can think for someone having the nerve to make such a demand.
In general, I think it is a bad idea for those of us who are not security professionals to go about reinventing the wheel. Depending on how important your workbook is, you may want to research external access controls that can't easily be circumvented.
I have a folder full of software that can hack just about any Excel workbook. Your user's password wouldn't remain secret very long, if a determined person has ten minutes alone with your workbook and Google.