Grandparent trying to protect grandson

My sons girlfriend is physically, and mentally abusive. My son accepts her behavior, and is determined to allow my grandson to be in this environment   I am fearful for my grandsons physical and mental safety. My grandson is 4 years old. My daughter in law wants to keep him away as well but most courts see this as an act of jealousy. Is there any action that I can take to prevent my son from subjecting my grandson from her violence. My son says I have no rights and can not keep him from doing this. Her last out burst was when she punched my son in the face and choked him. My grandson has experienced her verbal out burst where she cusses and screams at my son, and throws his clothes in the trash.  She did the same thing Christmas eve morning, and one episode she was yelling and cussing my son while he was driving and she ended up ripping the door handle off his truck. My grandson was so scared that he was screaming and crying. She has had domestic abuse charges on her record. Help!