Well, experimentation at this age is pretty common. And I would image his parents are also aware of this.
What you say to his parents really depends on how good your relationship is with them. If you say anything, I would keep it very nonjudgmental and on a kind of "hey, I noticed this, and just wanted to let you know. Is there anything you would like me to do if it happens again"?
If you do see him and the cat doing the same thing again. You might warn him that cats can have flees and you don't want that down there. Also cats are very playful and have very sharp claws. You certainly would not want the cat to take a playful swipe at anything down there :)