Start by telling your friend what you have told us here and it never does harm to be honest and say that you really have no idea what you could possibly do or say that might help her, but if she knows of anything she would like you to do, then she is very, very welcome to phone.
And keep in touch. It can be very difficult for the bereaved to phone or write saying that they are miserable, furious, want to talk about their loss.
If you get the sort of answer many of us do, "Oh, I' m all right, life has to go on" just acccept it. The day will come where she will be able to tell you if she isn't coping
If appropriate, tell her that you are praying for her,