My childhood best friend is now dating my ex who is a dangerous person, what should i do?

So last year I dated a guy for a couple months on & off.. who at first treated me like a queen, very affectionate, attentive & always bought me gifts and flowers and food BUT, he would want to be around me 24/7. At his job, he would call me every half hour and would just tell me to hold on and would keep the phone opened without being able to talk to me, & if i hanged up he would argue with me & ask who I was with. I got so tired of it that i broke up with him & he got so depressed that he quit his job! So i stupidly took him back.. So during any conversation If I told him I would call him back, he would call me back in less then 30 seconds NO LIE and make up an excuse like oops it was by accident, or im calling you to say that im gonna be busy so dont bother calling me when your done.. SO WEIRD!!. So besides being very insecure & jealous and a DRUNK, he didn't want me to hangout with friends, to dorm at college or to go out. 1day he showed up to my modeling competition (which i won so I was going around saying thank you to everyone as the camera followed me) & he shoved me, hit me, pulled my hair & threw a drink on me. So I dumped him. said he changed & went to church & counseling & he loved me so I took him back but not even a wk later I left him for again his embarrassing jealousy scenarios in front of people. I happen to find out, a yr later my childhood best friend (who i still speak to but we just grew apart) is dating him. She is a church girl (a virgin), never had any past boyfriends & very nice energetic girl. I dont want him to hurt her, i am very afraid for her, her mom spoke to me & has told me she is afraid herself. But shes 18 & nothing can stop her. I dont want her to think I want him or anything but i need to prove to her he is dangerous. HE still tries contacting me through my brother. I now have a daughter & boyfriend who i live with & i truly want nothing to do with her, but I dont know how to approach her about this. Im sure he lied to her about why him and i broke up (HE IS A VERY GOOD LIAR) so she might not believe me. I just hope to god he does not hurt her. =(