my gf parents?

my friend told me that my girlfriend dad beat and punch my girfriend mom i was afraid and hurt when her dad give my gf mom a black eye but her mom never say anything she always lied when i went to her house she said it was stressed couldn't sleep. her dad is heavy drinker and smoker he always drunk everyday when late night comes my friend told me she heard argument on their home and dishes was thrown out and stuff was a messed. my question is how do i stop my gf dad of being that person please help thank you:(

Report it to the police. Not much else you can do.