How is it the shops fault?
Fella walks in and drops off the parts and says "do what needs to be done".... He just gave that shop Carte Blanche to do whatever they wanted!

What he needed to say was, please disassemble, inspect and in X number of days I'll return to discuss the plan to repair.

Upon returning, had he decided to also replace the valves, it would be his option. As the owner of the parts.. he should get the final say.