There was a sequence of events which caused this, it is not just one man's fault.

The first cause was whoever installed that sprocket incorrectly. The second cause was being driven that way over that extent of time, allowing the chain to wear on things and become accustomed and worn to the irregular stress pattern, probably causing uneven stretch from one side of it to the other and causing it to wear more on one side. The 3rd cause was failing to tell the mechanic that it WILL be replaced now regardless of his opinion because we all know how parts that have been run in compromised situations are now compromised parts. Lastly, straightening the sprocket disturbed the chain, and now reset it to the original wear pattern, which it was not used to, and now placing a new stress pattern on a likely weakened chain due to improper wear, aside from now being made to want to ride out of the grooves it had most recently worn into the tensioner pads.
What I'm saying is, it was damaged, it was weak, and may or may not have lasted a while as it was had it been left alone, but it was only borrowed time, and halfway fixing it is exactly just that.

The circumstances suck, and you might be able to convince them to absorb it, not likely, but you might be able to get a bit of a discount at least. That mechanic can suggest things, but ultimately it is the owners responsibility to choose the work he does/does not want done. The true realistically liable party for this happening was the one that created the initial sprocket problem.

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Chris
64 190D R.I.P.
80 240D W/617 engine -for sale
82 240D -for sale