How do I help my son's girlfriend?

Hey just want advice on certain situation, my son has been with his gf nearly a year. Him 15 and her 14... she came camping with us for 4 days and 1 of the nights she broke down saying her poppa keeps gropeing her and it makes her feel really uncomfortable (it's her step poppa) her and my son have found teen porn on his computer aswell, poppa and nanna own alot of land and her parents built their house on it so theres no getting away from him. I told her to tell her mum exactly what she told me but she said she knows they will not believe her and she does not want me to say anything... I have met the mother and i get where this girl is coming from. But now I'm in a situation wherw idk what to do? I'm victim to SA and know if he thinks hes getting away with it its gunna get worse.... where do I go from here?