I don't know about who should pay the bill, but I would definitely make sure to report the dog who was already involved in an attack. My sister's dog was killed last year by a dog who had just been adopted by the neighbors. The dog had to be put down, because it had attacked another animal and was deemed vicious. The thing that made me the most angry was the owners who were supposedly "helping" the dog. If they had have been responsible, both animals would be still living healthy, happy lives. Instead, they managed to "rescue" a beautiful dog for just a few days before he was put down.