3rd party security for Excel

Hi all, this is my first post here so hopefully it is in the right area!

I work in IT myself but I am not a heavy Excel user in my field so my knowledge in this area would be intermediate at best.

I'm fielding a question for a friend of mine who wants some feedback on Excel security. He has developed a program which can be used by people to help cost out projects by allowing them to enter parts, materials and quantities and then it works out the costings. There is a simple front-end interface and of course all the hard work is done by Excel.

He is considering selling this as a product and has had interest already in the relevant industry but obviously he is concerned that potential parties may try and examine the calculations involved in the back end (whilst in trial) and then try and duplicate the program themselves instead of purchasing the product. He is familiar with the in-built security within Excel, locking and password protection etc but he is wondering if there are any 3rd party tools that someone may have used to further improve security.

If you have any suggestions or pointers in the right direction that I could pass on that would be great.

Many thanks in advance!

Thanks for the suggestion Pike. Would you say this is a complex process?