I seen this on the news this morning. I am so sorry!
I would just try to be there as much as possible for your granddaughter and she is going to be dealing with her emotions and loss while Im sure also trying to be there for her friend during this hard time. Your granddaughter is getting a double whammy dealing with both and as we know as adults its hard enough dealing with it ourselves let alone being there for friends and at 12 it hard to know how to do it.
I would try to give the family a meat and cheese tray and maybe one for your grand daughter also. I know when I have dealt with grief I never felt like eating but would "pick" at the meat and cheese trays people would bring over.
Hopefully some other mom's can help with some good age appropriate books that help with grief that would help the girls. Even going into Barnes and Noble or some other book store could help lead you in the right direction.
Right now I think lots of hugs and your shoulder and an ear is going to be the best thing for your family. Again Im so sorry.