I wouldn’t be at all surprised if the same people who read his essay also read the original-- adcoms tend to deal with the same geographic areas.
Wouldn’t it be a shame if the adcom either recognized it or realized that it was a familiar theme?? Or if it was run though a program that picked up on an overwhelming percentage of similarities?
At the end of the day, there’s nothing you can do about another person’s poor choices. You’re not Jiminy Cricket, sitting on his shoulder telling him that his conscience should be his guide.
Let it go. It’s not your issue, it’s his. If he’s old enough to apply to college, then he’s old enough to know right from wrong. You can voice your concerns, but it’s not going to help; you won’t be telling him anything he doesn’t already know.