Well, I like to believe that any creature that is treated with respect is friendly. However, with all of the game-hunters that like to hunt and kill animals for no other reason than their ego, I can understand if they're a little gunshy (pun intended). Zebras themselves are very social animals and reside in large groups, called 'harems'.
They sleep standing up, and residing in harmes allows other zebras to warn the others bywhiningloudly if they sense danger or see a predator. The biggest threat to zebras are habitat loss and hunters that want their skins. That being said, they're also very territorial.
If they spot a human in their territory, they're more than likely going to feel threatened and they're either going to attack or flee. With all of that in mind, it's safe to say zebras are not friendly to humans.