Don't do this just before she goes to bed. Do it in the morning. Wake her up gently. Make her breakfast. Don't plan anything for the rest of the day. If either parent works, have them get the day, or at least the morning off. Explain what happened to her friend. Be with her for the rest of the day. Do some painting. Watch some familiar, loved movies. Go out and do some gardening. Let her be alone when she wants to, always let her come to you when she needs to, even if you are busy.
http://www.amazon.com/Miss-You-First-Look-Death/dp/0764117645/ref=pd_sim_b_11
This is a beautiful book to address grief.
A script for you, if you can't find a book:
"[Daughter's name], I want you to come here and sit with Mummy and Daddy. We both need to talk to you about something that is very sad. I am so sorry to have to tell you this. Your friend Millicent died on Monday. She was in an accident. She was died in the accident." And then answer her questions as honestly as you can, without being too blunt.
(Now, personally, I would not tell your daughter than Millicent was killed in a car accident right now. I would probably ask a child therapist about this - only because for your daughter's first experience of death, it is a very chaotic one. Cancer and illness, we can explain to children as something that will not "get" them. But car accidents, we can't. If your daughter asks what kind of accident, I would say 'with a car'. And then you can teach her about being careful around roads, and not to walk on the side of highways, etc. Just say 'a car ran into her'. And then you can avoid walking beside highways for a while. And then, once you've consult with the psychologist, you can explain about car accidents, and how they are very rare, etc. But my fear would be that, because this is her first experience of death, she would become afraid of riding in a car, and that is something that you would need to discuss with the psychologist before you approached it.)
In a few weeks, it might help your daughter to see Millicent's parents. It depends if they are open to this, but it could be a chance of healing for both your daughter and her parents. To share in their grief, although it will be different kind of grief. However, it might be too painful for them. (I would say to them that you didn't tell your daughter that Millicent died in a car accident, because you were afraid she would be afraid of driving in vehicles, and that you told her Millicent died "in an accident". I don't know if they will understand, and if they do not, it might be better if they don't see her. But it is possible that they will understand, even if they react badly at first. Explain that you are talking to a psychologist to see how to tell your daughter in a way that won't make her afraid of traveling by car, but for now, she doesn't know exactly how it happened. I'm sure they will understand that.)
Another idea is to invite a child in your daughter's class who she likes and is of a sensitive disposition. They will be able to help each other work through their feelings.
Share your own stories of death. Of parents, of loved pets, etc. This will help normalize your daughter's experience, and show that feeling grief is normal, but you do eventually feel "okay" again, even if thinking about it still makes you sad.
I have read your other post, and I feel like you are handling this very well. You're doing such a great thing here, being honest with your daughter and being there for her. Thank you for being such a sensitive and loving parent.
Stay safe. Tell your daughter you and her mother will be there for her. I feel for you. I am so sorry.