Ok, here's my take:
Not all crappy parents are addicts.
And not all addicts are crappy parents.
Not sure why you make this about his drug problem. He's a sucky dad, doesn't really matter why.
Either way, it is what it is. It is NOT a reflection on her, nor is it a reflection on YOU.
You should neither stick up for him nor bad mouth him. She has eyes and ears and a heart of her own.
She will be ok. YOU will be ok.
I am not sure why you "do everything you can to foster that relationship". It is not a thing within your control. Sorry.
I have explained things to my own daughter, who was 9 when we split. Look, he loves you as best he can. This is who he is.
Over the years (she's almost 17 now) it actually became something of a standing joke. We all know perfectly well that when Dad says he's gonna pick them up at noon, the chances of that actually happening is about 50/50. They'd rather be home anyway.
Again, this is who he is, I cannot change him to suit their needs.
Keep a sense of humor, and keep her very close to you.
:)
Allow her to react to it in her own way, and then talk about it.