Replying to Microsoft Excel Help

so my roommate has created a database in excel that he wants to be able to let clients use. however, he cannot figure out how to keep those clients from seeing his formulas. he has tried using a series of workbooks (A, with the formulas stored in A, to B to C), so that the client never has immediate access to the formulas in A. but for the data to flow across the workbooks all three must be open which obviously won't work. he has obviously also tried locking it but a simple copy and paste of the workbook into a new workbook can void the lock. so I'm wondering if anyone knows of a way to let someone access a workbook without seeing the original formulas. +k for help