Unfortunately for everyone, it sounds reasonable. If this shop is not
familiar with M-B cars they didn't know how to do it but if it is a M-B
shop then someone forgot to tighten one of the belt driven components.
Either way, a belt came off its pulleys and whipped around in there
destroying the other belts etc..
This shop made a mistake but they're not lunatics, just sloppy. And
they're now correcting their mistake.
Upon the car's return you need to check that the a/c cools, that there
are no oil leaks and that the engine oil and the transmission fluid are
at their correct levels - between the notches of the dip stick - not
above or below. Check transmission oil when it's HOT, engine idling,
transmission in P, car parked on a reasonably level spot.
The same could have happened if the belts had not been changed and an
old belt broke and so came off its pulleys.