If the story is exactly hoe it happened, and the dog was known to be untrustworthy around kids, all the blame should go the owner. I have 3 dogs. they are all very friendly. and if people want to pet them at the park, I allow it, but I hold on to the dogs "collar" (not leash) just in case. my dogs have never snapped at anyone who didn't provoke it, but if I allow people to pet my dogs I will take every precaution, even unnecessary ones.
I had one of my dogs attack a person before, but in the dogs defense, this person (who my dog was not familiar with) approached my son a little too quick so she ripped into him.