You did the correct thing by trying to help her and show your concern for her by trying to get the authorities involved. It is sad because the teenage son is seeing his father (assumption made here) beat the crap out of his wife as a normal thing to do and will probably pass it down to the next generation.
Unfortunely the only person who can make a real difference and stop the cycle is the one who won't remove herself from the abusive inviroment she is in.
Sure, It would have felt good to beat the $hit out of the pu$$y that hit her, maybe it would have shown his son of a potential consequence to hitting a woman, but in the end you could have been arrested for assult and she would have gotten it worse at home
A sad situation for sure