Bless you for your concern, but sad to say, the law says the owner of the dog is responsible to keep it under control at all times. The woman who shot it is lower than snake poop, but the kid who ratted her out is not what one would call a reliable witness. His testimony would be considered hearsay...have you considered telling the owner of the dog what you know? Sigh...odds are, he is just as much a moron as the cretin who shot his pet. If he cared, the dog would not be running loose.
If it were me, I would write the dog's name in big black letters on a fist size rock, then pitch it at her car windshield some dark night. Owner letting the dog roam is wrong, her shooting it is wrong, by smashing her windshield you are also wrong, but remember, although two wrongs don't make a right, three do.
And since the dog's owner is so carefree, perhaps a call to animal control is in order. You can catch the dog and hold it for pickup, and contact them, and tell them you "heard" someone shot the animal. If the dog's owner is so (censored) uncaring, he doesn't deserve to be called the owner.
Shoot...(no sarcasm intended) I wish I could offer more advice.