Good for you that you educated yourself on addiction. You sound in control of what you want for yourself and strong enough to follow through with your boundaries. Understanding that it is only he who can make the decision to get clean and stay that way is a huge step in my opinion for anyone living with active addiction. Stay true to yourself and your needs.
As for his daughter, I really can't offer any advice, it's definitely a tough one. So sad that both her parents seem to be unreliable. You're right, you can't really do much for her without having legal custody or guardianship. I was thinking that maybe there is a counselor at her school who could offer advice, but once you do that things would probably be out of your control entirely because she/he would have to report any abuse or neglect she/he knows about, and then the system goes to work (child protective services, law enforcement, etc). Maybe the only thing you can do right now is think about what you are willing to do for this girl. What I mean is, if it comes down to it, would you want to take legal responsibility for her? Maybe being a foster parent or something like that.
Sorry I can't offer any real advice or solutions, I just want you to know that here you are not alone. I hope someone else will have some words of wisdom to provide.
Sending prayers your way.