But are you sure the bolt is from the water pump change? Any other repairs since then? Does this bolt match any other bolt related to a water pump change? Any fresh miss matched bolts related to the water pump change? Is it corroded or pretty clean? If corroded, it was in there a long time. The impeller break is fresh so there is no doubt as to what happened.

Did you pay for the WP change a year ago or was that the prior owner? This could matter for any sort of "warranty". What does the receipt say for warranty duration?

The point is that it will be difficult to prove one way or another so gather as much data as possible. The shop ( RE evil business ) always comes out the loser as courts side with the consumer if there is the slightest doubt.