I know that the advice that you're looking for is regarding your own daughter (completely appropriate, of course) but as someone who lost my own mum when I was young, I just wanted to point out what a special, wonderful place you can occupy in your daughter's friend's life.
My mum's closest friend still sends birthday cards and comments on my kids' pictures on facebook, and even that little contact means the world to me because it is a connection to my mum (and I'm nearly 40!).
You would be amazed how much it can mean, even years later, to have someone who really knew her just say casually "oh your mum would be so proud today / would love your fiance / would laugh so hard at that story / would want you to follow your dreams" etc. etc.
Because your daughter's mum is your own dear friend, you can be someone who can talk fondly to her about her mum, remember times that you spent all together and, if the girls remain close and you remain in the daughter's life, remind her as she grows of how much her mum loved her, how lovely they were together, things like that.
I would imagine that your dd will just need a lot of reassurance that it is very unusual for kids to lose their parents so young, and that you are very healthy (hopefully this is the case!) etc. Please tell your dd not to be afraid to talk to her friend about her mum after she passes - she will need someone at her own age and level who remembers her mum for the lovely person she was, not just as "xxx's dead mum who must not be mentioned" - it is painful to realise as a child that other people think of someone precious and irreplaceable to you as something scary or embarrassing that shouldn't be talked about.
I'm so sorry for all of you.