I'm sorry but i fundamentally disagree, specifically with a dead or sick Koi. Yeah, they may know about it, they may not know about, enquiring about it should provoke a reasonable explanation, if it's a dead Koi they have no idea about they should at best thank you for alerting them!
As a responsible hobbyist surely everyone should point out dead or sick fish?
If the dealer couldn't give a toss when you tell then there are questions to be asked!