50lbs of dog is a bad opponent for a human, 2 of them and humans lose that fight fairly often. Some of the dogs in the video I saw were that size and larger; so if the monkeys can harass and herd them effectively then I'd expect them to be able to do it to a human as well.
A human prepared to fight and ready to start killing things is a completely different matter; but I don't think these critters have encountered that yet. They're probably about to.