Who's right, who's wrong?

Two of my best friends are threatening legal action over this matter, and both of them could be wrong. Before this gets any worse, could some body please help...

Jack goes on a weekend camping trip, and asks Sally to babysit his Miniature Pinscher dog. Sally always has a good time with the dog, and agrees, as she has babysat her before without any problem. Everything is going fine, until Sally goes to sleep, and wakes up in the middle of the night to a horrific crying dog..It has broken it's leg jumping/falling? off the bed! Sally rushes the Min-Pin to the vet, and pays for its xrays, pain killers etc. The vet tells Sally that the dog needs surgery that will cost $2500-$3000.
Sally doesn't know what the owner wants to do so she contacts him.
He is hot-headed naturally, and says that she should take responsibility for her actions, and pay, or else he'll sue her.

Is it relevent that the dog has already broken this same leg a few months prior to this injury?

Is it relevent that the vet said that this type of injury is very common in Min-Pins because of how small and skinny they are?

Does Sally need to pay the surgery fee? If so, is she to blame because it was under her care?

[annjd]

She is not responsible unless her negligent act caused the injury--and we don't see that here

yes those facts are very relevant

it would seem he has no case