While it's true that an auditor can look at an excel spreadsheet, my experience is that they'll never really understand what's happening for anything more than the most basic spreadsheets. It's far too easy to build a spreadsheet that's completely inscrutable, and you end up with an audit that's very shallow.
Then the auditor should ask you to prove it another way. You may think you hoodwinked them, but don't be so sure that they didn't build their own model to check your work... They do this all the time when people say 'I don't know how it works, the ERP calculates it.'
