Oh Zak, I am so sorry what you have been going through. You are a strong person to have lived with this and taken care of yourself and your kids through it all. Your girl is messed up to say the least. Sounds typical of addicted people though. Really sorry about the best friend thing too. He's just as sick as she is.
I just want to say that:
(1) Yes, she CAN make it without you there and she HAS TO-We ALL have to "make it" on our own. You come into this world alone, you go out of this world alone.
(2) I wouldn't tell her a thing. You're not going to get anything out of telling her and neither is she. My experience says it's only going to cause more drama for you and the kids. And only make things worse for everyone. Just go on living your life and making the healthiest decisions for you and the kids, and leave her to work on herself.
(3) You being in her life holding her up and putting out her fires is only hurting her.