I'm not convinced it is a hoax. While the hoax video is quick to point out the water balloon or tennis ball theory I don't buy it. Based off the size of the object traveling through the air, that would have to be a very large water balloon. Also... the hoax video points out that not much water would fly from a melon impacting... he apparently has never chunked watermelons or shot melons before. They are juicy.
Either way, getting hit in the face with a sling shot device with any object would really piss me off. If you watch the interview video with the host of that show, she explains that a large amount of time went by right after the incident as they had medics check her out and get her calmed down. It wasn't until later that she approached her and told her that she must finish.