Prayers for the family, and for your daughter.
I know what she's going through, I lost my best friend right after high school, and he was like the brother I never had. Worst day of my life still the day I found out he was no longer with us.
Tell her in time, she will be thankfull for the time she had with her. And that one day she will smile when she thinks of her instead of being sad....and be greatfull to the Lord for allowing her to be her friend.
My friend has been gone for over 25 years now, and I still enjoy and appreciate his friendship to this day. He's still the best friend I've got.