I’m not sure how to do what you want with customizing Mint directly, but a possibly simpler alternative solution is to just send two clearly distinguishable USB drives (e.g. label them “1” and “2” with a label maker or get two drives with very different colors) and tell him to install (unmodified) Mint from the first and then have him run a program you provide on the second after that’s done to make the other changes.