I've heard that story many times. Child dumping all the food in the tank, while owner is away. Another version, is when the fish-sitter decides to feed extra, and continues doing so, even when tons of left over food remain in the tank. That last one also happened to me.
The responsibility is on the owner, one must put out the exact portion that is to be fed, for each feeding. It could be in bags, containers, pill boxes, or something else to ration the food. Next, the food canister must also be out of sight (hidden even!) so the fish-sitter does not get tempted to feed extra (for some reason people LOVE feeding fish!).
But yeah, it's sad to learn the hard way . . .
Good luck with the survivors!