If you spend your time warning his current dating partner about his past how will you ever enjoy your life? You need to move on with YOUR life and leave him BEHIND. If he is as horrible as you describe, shouldn't you cut all contact with him?
Although you might think you are doing her good by informing her, really you are fueling something inside you. There is a reason why you feel this need to be so invested in the women he dates, perhaps spend some time finding out why you can't put him behind you and why you invest your time in contacting his girlfriends.
Everyone needs to make their own decisions, learn their own life lessons, and choose who to be with. If you contact her parents you could cause a family blow out that could sever a parent daughter relationship for ever. That might seem drastic but your "good deed" could turn this woman's life upside.
Please put him in your past and concentrate on your present. Living in the past is no way to live.
*Adalie