Honestly, your best defense in situations like this is to gather as much evidence as you can and then release it to the media. In the mean time, the next time the dogs show up around your mother, tell her to absolutely under no condition should she ever run. If she keeps her cool, they shouldn't attack. Right now it is important that none of you use any kind of lethal force except in a life and death situation.
Where was she at when she was attacked by the dogs? She might have stumbled across their "territory", and they saw her as a threat and acted accordingly.
Be sure to remind her that if they ever do start biting at her, to grab their lower jaw and hang on tight. Then tell her to start beating the dog on the nose repeatedly with her hand shouting as loud as she can "NO!" and then release the animal. If the dog comes back, do it again. She has to assert dominance over them, or they will never leave her alone.