I would also be very surprised if the cat tacked the dog. I agree about taking your song to the vet as soon as you can. If it was me, I would probably muzzle my dog, as I hate to see any animal killed. I disagree with the "a loose animal is fair game" philosophy that I've read in the past (not by you). The cat's life is worth as much as my dog's, and I would never want to see something like that happen again. I wonder if there's any way that maybe whoever is walking the dog would beat at the bushes with a stick or pole during walks to try to scare any lurking animals away.
BTW, inugrey and NeylasMom gave great advice.
I'm sorry that happened to you. It must've been very traumatic and it was an accident.