The only way I know to do this is with a macro. Hide the sheet as xlVeryHidden. That way the user can't see it in the sheets collection to unhide. To view the sheet they would have to execute a macro that would ask for a password (hardcoded) or even perhaps held on another 'very hidden' sheet. If they enter the correct password, have the macro unhide the sheet.
Of course you could protect the Workbook with a password, but then that will likely prevent them from doing a lot of other things you want them to do. Then again, maybe not, but it won't re-hide and re-protect the sheet when they are done.