Perhaps speaking to a counselor as a family would not be a bad idea. Not because of the imaginary friends - my niece (whose name is Audrie) has an imaginary friend named Audria and not long ago she mentioned Audria's mom and I asked what her name was and she said Momia. haha! Audria pops up with us often. Sometimes I have to fix Audria lunch (the pretend kind) and give her popcorn at the movies (again, pretend) So it's very normal for a child to have imaginary friends ESPECIALLY if they've just been through such a tragedy.
Family counseling may well be in order just to help them as a family learn to deal with this tragedy. I hope everything works out, best wishes!
I see you wrote this back in July, if you happen to read this - how is everything now?