Let the mourner know how you feel, don't presume to know how they feel. You can also show solidarity by acknowledging the difficulty of accepting their loved ones death.
"No one can know the loss you feel right now."
"Mary's life was ended too soon - it just doesn't seem fair."
"There's nothing I can say to ease your pain, but know that I am thinking of you."
Through God all things are possible, all hurts healed.