I have an experience similar to yours, but amazingly enough, he is not an addict.
My five year old son has a deadbeat father who has never seen him, talked to him, anything, ever. He lives around the corner from me. Trust me, I want it that way, it was hard enough in the beginning, but now that he is trying to get back into my life, I don't want him to. He literally destroyed me.
He has a new wife. Interestingly, she looks kind of like me, he accidentally calls her my name frequently, and he drives the same car I drive, which is not a common car. The new wife knows nothing about me, or what happened between us. Duh. He would have to be a complete dipsh*t to tell her what he did to me.
Anyway. She is trying to get pregnant. Why, exactly, I don't know, because he already has 3 kids, 1 who he has never seen, and he only sees the other two like once a week. I want to tell her what he did to me. I want her to know that I was also a stupid 18 year old who got pulled in by his lies and ended up being completely ripped apart, and nothing will ever fix what he did to me. I don't want some innocent girl to have the same thing done to her.
That said, I know I need to stay out of it. I want her to know what he did, but I don't want him to think that means I want him back or something stupid like that. It is going to hurt my feelings when I find out what he does to her, like guilt because I didn't say something, but I know that if someone would have said something to me like that when I was with him, I wouldn't have believed it. As hard as it is, some lessons need to be learned the hard way.
I know that things happen for a reason, and there is a reason why he did what he did and made me the way I am. I may not know what that reason is, yet, but he changed me. And no matter what anyone would have said, I would have been with him, no matter what, because I was blinded by my love for him.
There is really nothing you should do to save this poor girl. As sure as the sun rises every day, you know that he will do something to her. But in the end, it's going to hurt you if you try to get in the middle of it, because he is going to end up taking it out on you. Whatever she is meant to go through, she is going to go through, regardless of what you have to say or what her parents will say if they find out. You just have to hope that she is strong enough to get through it when it happens. I know that is not a good answer, but it is all I have, although I do know how you feel.