I understand that they have great faith in God. Why isn't it also possible that God has decided to call this girl to heaven for His own reasons and they are simply ignoring His will? I simply do not get why the automatic impulse is that God wants to work a miracle. We all know that young people die tragically all the time. Somehow, some way, this is also part of God's plan.
Have you had your child be declared brain dead? If not then you probably wouldn't get why those parents feel that way.
I don't believe in God, but I know if I was in their situation I'd be praying to someone/thing for a miracle to save my child.