Yes, but there is no percentage in it for him. It's like telling a random fat person on the bus that he's fat. Probably nothing happens, but there is a small chance he will fly into a rage, or will tell everyone that you called him fat (maybe blogging about it, with video!). There's a much smaller chance that he'll suddenly start to diet/exercise, but how does that benefit you? It's different if it's a friend/family member, or if you're hired as a trainer or doctor.