for your own sanity i'd not help her get a ride to see him. if something happened to her u would always regret you aided her getting there. if she asks i'd say, i have a committment, sorry. no need to elaborate to her.
you can be a friend to her but know that one that is abused has a hard time realizing they need to break away from the abuser. it's like a person captured who starts identifying her captor as her only savior. not logical but i've been there.
she will need to make the decision and only she can do it. i know you are concerned but she has ignored your observations of her volatile situation. it may take a bad turn but you have no control of these dynamics. add alcohol or drugs to the mix and the danger heightens.
the way i feel you can not get dragged down is to detach yourself emotionally from their drama. you are not being unkind to do so. it is self preservation. hopefully she'll realize she has to preserve herself too.