its not a lesson he should ever be taught
the whole reason people take their cars to garages and get MOTs is because they expect the car to be safe roadworthy afterwards. same as how if you took your PC into OCUK for a repair/upgrade you wouldnt expect it back with a fan disconnected or the psu set to 110
the garage has made a big mistake and they should be looking to claim from their insurance to cover the damage they caused
last year a lad at works wheel came off and he ended up in a ditch upside down on the way back from his MOT , the garage originally tried to pass the blame but he won once it went legal. the garage got off lucky tbh. someone could have been killed. and the same in the OPs case.
No, I totally agree but unfortunately these things happen. I guess it would be possible to prove negligence and ask if they perform a vehicle check before a car is handed over to a customer and try and persue it that way. It won't be easy though.