I Have A 4 Year Old Granddaughter Who Witnesses Domestic

i have a 4 year old granddaughter who witnesses domestic violence on a regular basis, and i have seen her cry for no reason, i'm afraid for her mental welfare.. what do i do? i've talked to both parents, her dad will listen, but mom, well i think she hurts her and terrorizes her i feel...nevermind, too many questions for me.. just want advice.