It is nice to hear about such wonderful, supportive friends. I worked for a Cancer Institute for a couple of years, and found that support and encouragement from friends, family and coworkers was one of the most important aspects of a patient's recovery.
A couple of suggestions I have: The American Cancer Society has wonderful booklets for eating while undergoing Radiation and Chemotherapy. They have specific recipes that are very helpful.
Dr. Susan Love has a terrific book out (Dr. Susan Love's breast book) that is VERY comprehensive. (Not recommended for the very anxious patient - too much information can do more harm than good). This will help your friend with lab results, doctor visits, and general questions.
Some people may recommend to you the "Dr. Atkins Breast Cancer Diet". Please read this book with caution before you go further....
Your friend will go through very normal emotional stages. We had a "healing coach" at our cancer institute that is starting a national campaign to have more healing coaches around the nation. She was wonderful with the patients. There are so many issues with cancer, and it helps to see a counselor who specializes in these issues and stages.
Finally - some little things that I think seem to "help" -
1) Rides to appointments - it is so helpful to have a friend/second pair of ears to listen to the doctor's and nurse's information
2) Massage - by someone who knows about cancer pain
3) Reflexology has shown wonderful results with Cancer patients
4) Take care of yourself and the other caretakers around your friend.
Hope this is helpful.... I think it's wonderful you're ready to help.