First off tell your friend to keep his hands to his own plants!! That being said, the plant will continue to try to send water and nutrients throughout itself even with a damaged leaf. Some say remove damaged leaves if they are beyond 50% damaged, while I have seen (albeit only a few) people say to cut leaves in half (damaged area off). My thought is even when it's damaged, it's still photosynthesizing on the living tissue, and the damaged/browner part that's dry, won't send or receive anything. If you cut, it now has to heal again. I only remove leaves that either are dead and about to fall of anyways, or if the canopy is so thick, that everything underneath is completely shaded. And even then it's a bate minimum to allow light through. Leaves are more than solar panels for plant life. They allow breathing and transpiration (if thTs the technical term for it ).