How do you explain death and dying to a 9 yr old?

My nephew has a best friend Willie. Willie is adopted as is his younger brother. Willies mom is a single adoptive mother and has been raising these two since birth. Well, she has been battling cancer since 2006. In the last 2 months her cancer has spread to her brain and she has had over 100 rounds of chemo and radiation. She is starting interferon treatments as well. She has been told she will be gone by christmas. She is a woman of strong faith, but has finally come to the realization that her time is coming to an end. Not only is this hard for our family as we all grew up together, but my nephew has no idea how to deal with this. It is unceratin where the boys will go once she pes, so they will probably not continue going to the same school together. My nephew has no concept of death and is becoming very depressed about the entire situation. How do you help a child handle something like this?