Regretfully, I don't think there is anything they could have done while replacing the starter that could have caused the water pump to fail. For an example, I just got done doing a head gasket job on my wifes minivan, and the alternator took a dump on me right after I started the van when the work was completed. I know I didn't do anything that could have damaged the alternator, it just picked a really nice moment to fail. Since the car was leaking coolant previously, I think it was a coincidence that it failed when it did. The water pump on your car is about as easy as one can be to change. The part should cost around $30-$40, and labor should be an hour at the most if you don't have the ability to change it yourself.