If he was in anything less than a 40-55 gallon tank with no filter then I am not remotely surprised. They have an adult size of over 12 inches long and can easily live to over 30 years when kept correctly. They eat a lot, shit a lot, and basically need an external filter to cope with all the crap and ammonia they produce. Plus you need to do weekly 50% water changes without fail.
Did they tell you any of that at the fish store?
Keeping even the most "basic" fish is no walk in the park or cheap effort. Goldfish require exactly all the same equipment and treatment as tropical fish, just without a heater.
Can you tell I advise on a fish forum?
Haha.