It seems your friend has their answer. Yes, the industry should be regulated, but if your garage says you need new parts then you can ask up front to see the removed parts once replaced.
There are plenty of business models where parts are replaced without an obligation to show them, boiler repairs, air conditioning, electricians, plumbers, washing machines. In fact, at work when I talk about a lazy approach to fault finding, where part after part is replaced instead of identifying the root cause, I call it a “washing machine repairman” approach (no offence to washing machine repairmen).
If I have a pressure switch replaced on my boiler or A/C system do I then ask for the old one for testing? No, ain’t nobody got time for that. But then I wouldn’t have it replaced unless there was a self evident fault, so perhaps this is the lesson, always witness a fault before repair.