My heart goes out to you, your friend, and her family. I'm sure she is blaming herself right now, thinking that if she would have just held on a little longer, this wouldn't have happened. But in all honesty, it probably would have happened even if she had not cut those ties.
I am constantly worrying about my boyfriend, who is addicted to and abusing Oxycontin. I used to cry, beg, plead for him to stop using, but now I just remove myself from the situation when I find him using. I leave the house, get away from him so I don't react in an unhealthy way, but the entire time I am gone, I worry that he's shooting up. I panic, imagining myself coming home and finding him dead on the bathroom floor. I feel like I need to go back and check on him, like I'm responsible for his safety.
I told this to my therapist last week and she told me that he is going to use and abuse whether I am there or I'm not. The truth is that an addict is going to keep using and abusing drugs regardless of who is there for him/her. My bf uses when I'm there and he uses when I'm gone, and I've realized that if something happens to him (and I pray every day that it never comes to that), IT IS NOT MY FAULT. He is in control of his own life and there is nothing that can change that. Nothing that I say or do will make him stop, and I shouldn't feel responsible for his well-being. The only person who I can take responsibility for is ME.
If your friend would have remained a part of her brother's life, it wouldn't have made a difference in terms of his drug addiction. She did what she felt was necessary to protect and take care of herself, and she should not feel guilty about her decision. He chose to use drugs, and there was nothing that she could have done to stop him from using.
I also agree with Outtolunch about your friend seeing a grief counselor. Seeing someone with experience in this area may be the key to her recovery from this awful situation.
I hope this helped. Good luck.