The customer is always right in this situation. Do NOT yourself claim that it is not possible, say you don't know. DO show him the manufacturers response. Ask the customer what he wants you to do. Compensate the customer in some way--say a free ap next year. Its a cost of doing business. If you handle this right, professionally, he will give you referrals, if not, he will tell all his friends that you are incompetent.
Actually you are right--it is highly unlikely. Fish die all the time, too many fish for the size tank, warm water conditions, low oxygen, disease, and sprays of pyrithroid products (as pointed out above). Why would the water get clear? Hmmm...maybe they used an algicide...and it killed the fish. Swimming pool algicides probably do not belong here.