Since there is no answer yet.
Today, I had a similar problem with the tablet of my mother-in-law. So I was searching also the Internet and didn't found something except for this. I had a look into the files. It turned out: All these files were created between 10. August 1pm and 10. August 6pm. Before this everything is okay, after this everything is okay. So I figured: Between this time range the tablet was lying around and something tiggered the shoot-a-picture-button. Since the camera was facing the bottom, the pictures are black.
I guess this is the problem here with yours too. If you have a look into the create dates/times, they are very closeley. Your sister just shoot a thousand pictures of nothing.