Hi Dianna,
I don't know the age of the child in question, and that actually would help a lot. For most young children, though, context is helpful. Perhaps that was given to this child in your post, but you didn't mention it. (and I understand that your friend is rather raw, too. I have empathy for her; a sibling's spouse recently passed in our family, and it's been a sad time.)
Children need the idea of death grounded in the context of life. Death by itself can tend to be huge for any sort of person; death in the larger context, centered in its purpose with life as a whole, the yin and yang of it all, can help neutralize the shock of the idea.
That every living thing should die is part of life, yet what children might need is a gentle review of average cycle of life. People are born, they are babies becoming toddlers and then big kids, and then the become teenagers and grown-ups. They might have children. They often live to do and see a great many interesting things, and they have people in their lives whom they love and love them. And at some time, they will die.
If it were me, I would stick with the situation at hand. For every family, the causes of death are different, but just stick to what's present for now. We don't want to give our kids *all* the potential causes of death, which can greatly overwhem a child. We just want to give context for what's happening here and now. We can also let our child know how much their departed dear one loved them. Sharing memories, building a small altar or memorial (in case the child wants to draw pictures or leave treats for them) and just remembering them in a happy way is helpful and will encourage kids to share their feelings.
Kids do hold onto this stuff for a long time. I once knew a child who, at age 6 or so, grieved that her grandmother had passed before she was born, and that she hadn't met her. Children are beautiful and honest in this way, so be as open as you can. Don't try to smooth everything over or distract them to help them feel better. Empathy--"I miss them too. You were very special to them/They were special to me"-- is often all children need to hear.
And let your children know that you will be with them for a long, long time. They need us so much; if we suggest a life without us at a young age, that can bring on a lot of anxiety. We can't control what life brings, but as today's featured post, "The Perfect Mom", explained, we want our kids to have the "regular" problems with us. We don't need them to grow up worried that we are going to die tomorrow!
All the best,
H.