It's hard to find words of support when someone is grieving. We want to relieve their pain but there is no way that we can do that. Facing the death of someone we love dearly is painful and there is no way around that.
Talk to her. Listen to her. Hug her. Just being there for her will mean more to her than anything.
Although I don't know what the future holds for my addicted son, I recognize that death is a high possibility. Addicts don't fear death. They fear life without drugs. I'm sure that your friend recognized that death was a possibility for her brother. That doesn't make it easier. Her guilt feelings are a part of the grieving process. Time is what will resolve her guilt feelings. She didn't cause it, she couldn't control it and she couldn't cure it. Her love, as great as it is, was not that powerful.
When someone dies that we love dearly, we want to talk about them. We just want someone to listen. We want to know that someone cares that we are hurting so badly.
Your friend is lucky to have such a good friend in you.
gentle hugs
ke