My mom died after an illness when I was a few years younger than this girl. I remember getting books from caring friends and relatives after her death. Honestly, though, these books just reminded me of what had been taken from me and what made me different from other young people and it was a while before I opened them or started taking their advice seriously. But I still have a couple of them on my bookshelves today.
Probably one of the most helpful for me was Social Savvy by Judith Re. A lot of it is etiquette for certain social situations, but there are chapters that cover how to deal with different types of people, money, dating and that sort of thing. Teenagers think etiquette is an old-fashioned concept. But you're not meant to use it as a rule book. I think the point of it is more to show a young person what is expected out in the real world and how others might view you for your actions (or inactions). The sort of thing mom would teach you as you're getting older to prepare you for leaving home.
Hope Edelman's Motherless Daughters is a beautifully written book and is commonly recommended for young women in the same situation. For me, it was like rubbing salt on the wound and I still have a hard time reading it. I mention it because if her daughter has a strong sense of who she is, then she may take some comfort in reading this book without feeling like she is doing something wrong if she deals with things differently.
I feel for your friend and her daughter. It's a painful road ahead, but it does, very slowly, get easier as time progresses.
It's wonderful that your friend wants to be honest with her daughter about the seriousness of her illness. It's a brave and very selfless thing to do. In order to try and protect me, my family never prepared me for the inevitable and I think that caused a lot of problems later on.