You’re taking the short term view. I see it differently. I 100%, fingers crossed, rubbing the rabbit’s foot HOPE that he gets rejected by every school that gets his plagiarized essay.
If his ethics and morals don’t match his academic ability (I assume this b/c you think he has an otherwise decent chance of getting accepted), then I want him to learn the lesson now.
If he’s indeed that academically qualified, he’ll won’t get shut out from colleges.
Your fault? How is that? Does he have no moral compass of his own? Does he ask your permission to do right and wrong? He already knows it’s wrong. As a friend, you SHOULD blankly tell him it’s stupid, he can get caught, and that it demonstrates his moral bankruptcy. And then you’re free of it as far as your friendship goes.
Frankly, one could question your moral fiber. Why would you stand aside and allow wrong to progress. Tell him: you didn’t invite his confessional but if he intends to proceed, that you’ll spk w/the HS guidance counselor.