Crankshaft/serpentine belt?

We recently purchased a 92 Eagle Talon for our son. The car had just come
from repair having fuel injectors and a new alternator installed. The engine is a replace engine (mitshubishi) with less than 100,000. Standard Shift The
charging system was not working properly, so we called the garage that had
done the work and the mechanic said that he indeed had not used an altmeter
to check if it was charging. We had the car towed to the garage as it was
not yet licensed. The mechanic called us back to say that the alternator
fuse was blown. We also paid as he said there was a fuel line leak and we
got an inspection also. While he had the car for repair we licensed the
vehicle. My two sons picked up the car and drove it less than ten miles
down the road at a maximum speed of 45mph. The car started to shake and
made a load noise, and then stopped running. My son coasted let it coast to
the bottom of a small knoll. They opened the hood, the car was steaming, and
the serpentine belt was totally loose(not broken or frayed)In fact, I think
the belt had just been replaced also before we bought it. The car was towed
back to the same mechanic and the only thing he has told us is the crank
shaft is shot.(Cracked or bent? I'm not sure of which he said as he talked
to my son who is not here currently. Others have suggested that the engine
went because the belt was off and the engine wasn't cooled. If the mechanic
had just been working on parts that required removing the belt, is it likely
that he is responsible for the engine failure