Computers are fast, if you haven’t heard.
The key is to obtain verifiable archive data that he can prove hasn’t been altered. In normal M&A diligence the acquiring company says, “thank you” for all summaries or rolled-up data they are given, but then “Now we’ll need to have all the archived data so we can do our own analysis”. You don’t spent millions or billions of shareholder money without doing your own independent analysis, never just relying on “trust me”. Ask anyone who has participated in corporate M&A, it’s a game of “you bet your job” whenever you make recommendations to top management about a target company, and they are relying on YOU, not on what you have been given.