How do you know that he is wasting his time? If he do it on his spare time only, you cant say that he is wasting his time. You can say he is wasting his time if you know that most of his time is used to hunt for airdrops and bounties. Regarding the rewards, there is no guarantee of earning money on both airdrops and bounties unless the reward is paid in coin that has real value and already traded in some popular exchange.
It's believed by everyone that airdrops are such a waste and likely most of them are. But you're also correct that if somebody only uses his free time.
He can do whatever he want with it but if there's another more meaningful thing that he can do with that time rather than doing in airdrops, it's all up on him.