We used to buy in signals from a company who did their work in Excel. I wrote some scripts to export the data and recalculate it in Python. Almost every month I found errors in their reports and had to ask them to fix it.
So I recommend you fight HARD to get someone to reproduce his work in a language that is visible and reproducible.
Which is why they will not be switching to python or R any time soon and even if they do, it will still have similar issues as the Excel version.
Just because you know how to use git history, diffs, etc.. to spot differences in code doesn't mean that is going to help the layperson.
>I think it is more because of your experience as a developer why you were able to spot and correct errors.
I don't feel like I spotted errors. I wrote a script to validate the data and the script told me if there were errors (there were).
Also the people who use Excel as a primary tool are not the type that write unit tests in Python generally. Or would even think to do something like that. That is my point. You, as a developer, would think of something like that. It's not that you couldn't write similar tests with excel (you could, in any .NET language). But that you thought of doing so.
In GGGP's case they almost certainly have no tests so I stand by the recommendation that they fight hard to get a validation system in place. Probably by moving to Python and/or a RDBMS.