Where to report a bad, dishonest mechanic

The $2,800+ bill was itemized. My friend checked the auto shop’s arithmetic–she added up all the amounts on the bill–and the total she got was $100 less than the bottom line on the bill. She went back to the shop and they refunded her the $100 overcharge.

I am not accusing the shop of “malice” in failing to perform the needed repair–but of incompetence of the first order. I accompanied her to the shop on her second return visit to find out the source of the leaking fluid on the driver’s side front hubcap, and I went with her to pick up the car and saw it exactly where we left it, and listened to Ronaldo’s BS explanation that there was nothing wrong, that the wheel must have been splashed by a passing car. I also looked at her itemized receipt, and I took a copy of said receipt to my own mechanic, who, as I say, took the wheel off and immediately found the broken caliper and leaking brake fluid.

All I want to know is whether anyone can suggest where to report the auto shop for incompetence and negligence. I am not in the least worried about being sued for “defamation” or anything else: Ronaldo and his auto shop is lucky we don’t sue HIM.