Maybe you could call the police and they can direct you to the right phone number for the animal control or spca in your area to report this abuse.
Maybe they can collect statements from people, such as the older woman who witnessed the dog being beaten. I haven't read your other post but it sounds like there may be another issue of abuse? I'll go and read it now.
But if that is the case, then there is definitely a pattern with these people and more animals will suffer down the road. It would be good if the authorities could be made aware of them so they have something on file should something else happen. I pray that nothing else happens.