Serpentine failure at that age is rare. If a seized tensioner pulley caused it that would have been apparent at the time the belt was being replaced and it also should have been replaced.
It is very likely that a failed serpentine belt snags the crank pulley and bits of it get left in there. If the old belt had bits missing they should have taken a basic precaution and checked the timing belt cavity for residue. Any such residue can get tightly wound up behind the crank pulley and not make it's presence known till later.
Many would replace the timing belt at this point on the basis that absence of evidence is not evidence of absence of damage to it and serpentine belts always try their best to have a go at timing belts.
IMHO they failed to check as they should have. The only excuse they could have would be if in fact the residue was from an earlier belt failure and the one they removed was intact - a highly unlikely scenario.
You are lucky in one sense that the belt failure didn't kill the engine before it ever got near the garage. It appears that their neglect undid your luck.
I have no doubt that expert opinion could be adduced to support your claim that they should have been more careful.
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