Hey doo doo happens. Next time you see him make him feel like an ass about it then move on. He did it because he is dumb not malicious.

If there is no flap at the slice ignore it and strop as usual. If there is a flap get some barge cement and put it into the flap and leave it open for ten minutes. after the wait when it looks like the glue is too dry and useless then it is the perfect time to close the flap. Carefuly trim away anything hanging and sand it with 220 sandpaper. You will be good to go.