My sister had a dog that got into a neighbours garden and attacked their goat, which was tethered. Apart from having to pay a hefty vet's bill, they were taken to court and had to provide evidence that the fencing between them had been repaired and brought up to an acceptable standard. I think the courts consider an attack on another animal as less serious than an attack on a human, and so don't usually make a destruction order. Of course owners are responsible for keeping their animals secure, but unfortunately dogs will be dogs and given half a chance some will attack smaller animals or poultry. My GSD will take a pigeon out of the air if he can catch them and I don't think I would trust him to bring the chickens in!
To be honest I think provided the owner paid the vet's bill and the cost of replacing the chickens and does ensure the dog is secure in future then you can't really expect any more. You can't blame the dog for doing what nature tells it to do.