How to give access to a user to change one Particular Workbook?

We have a situation:

As Is: Our end users in Production system can create and save a workbook in to their favorites.

To Be: I want to give access to a particular USER to change and save a particular WB in the same folder.

Note: He should have access to all WBs but could be able to change only one particular WB.

How can I do that?