I think the owner must have bred these dogs to fight. That does happen frequently, and when the dogs go off, the results are ugly.
"My daughter has a pit bull who is the sweetest dog imaginable. He is a big baby, affectionate, gentle, and well behaved. Because they have a reputation for being vicious, my daughter probably over disciplined him when he was a pup if he displayed any aggressive behavior. He is neutered, also, which probably helps to tone down the aggressiveness."
Virtually every time you read one of these stories, there is a mention of the animal having never done that before and being very sweet. What was her reason for buying an animal that powerful and potentially dangerous over another breed of dog that lacks the ability to kill? Why not get a lab, or an irish setter? Even if the risk is small, I don't see the point in running it when you have so many great alternatives.
Just out of curiousity, did she get him from a show breeder?