I have always felt that to know an animal is being abused - to know it as fact - is to be morally bound to do something about it.
How intrusive those measures grow I think then becomes a personal decision. (After believing for over ten years that I was destined to be a vet, I chose another career, strictly because, after helping out at a clinic, I didn't think I could restrain myself around abusive owners. I can just see someone bringing in an injured pet, and me "visiting" upon the abuser the same injury inflicted upon that helpless creature, then getting arrested, going to jail, becoming notorious.....oops, sorry, got distracted.
)
At the very least, I'd call the appropriate authorities.
If you are in this situation, StewiesMom, I wish you luck. It's not a comfortable place to be.