I'm so sorry to hear this.
I know there isn't much you can say to help her right now, she needs to mourn her loss. With time, hopefully you can help her wrok through "everything happens for a reason, and we may not ever be aware of what that reason is." I think that's a concept that many people apply (regardless of religous belief, i.e. God has a plan, it's nature's way, it wasn't in the grand scheme of the universe etc.)
Good luck as you walk with her through this, my thoughts and prayers are with you and her.