Hi T., just be honest. Cancer is just a disease that can affect everyone. Although predominant in adult women, Breast Cancer may be found in the young and men as well. Let her know there are different types of cancer and it can take a life, but when dealt with timely it can be under control as well.
Let her know her friend will need her support and understanding while she goes through this. If you are not familiar with it, research together.
My mother was recently diagnosed with Cancer of the colin and I worked hard on informing myself and preparing myself. I kept myself aware that I could lose my mother, but we would do everything possible to beat this. So far, we have beaten it.
Let your little one know, everyone is different. Her friend may choose to be private about her illness and may choose to not share her experiences, treatments, or feelings.
I will be thinking of your daughter and friend and say a special prayer as I photograph the many survivors here in Vegas on May 2 at the Susan G Komen Race for the Cure.