Why do these leaves look the way they do?
The leaves did not disintegrate. They were eaten by an insect with chewing mouthparts. It could be any one of several. If it was gypsy moth larvae, they feed at night. I doubt the guy was out with a flashlight. Or, whatever was feeding is done. No insects feed all growing season. For many chewing insects, they, feed about four to six weeks and then are done. They pupate and become whatever they are as adults.
By the looks of these leaves, the guy either did not look at the tree or has very poor powers of observation for weeks. This is not overnight damage.
If less than 50% of the tree looks like these leaves, it's no harm, no foul. Water the trees to help them put on new leaves but do not fertilize.