If it happened to me I'd have to write a letter and make sure the owner of the dealership received it. I'd tell your story then add something like this:
"If it were not for the watchful eye your book keeper I probably wouldn't visit your dealer again, nor would my friends. I look forward to our continuing business "
....or some other similar glowing comment about the honest book keeper.
Who knows, the owner might not be aware of what some of his sales people are doing and be appreciative for your comments.
Might even get an oil change or other coupon out of the deal too
