Nobody died when they ate of the tree of knowledge. 
They died when they disobeyed the command of God. The fruit didn’t kill them, the disobedience did. The Serpent was crafty, that is, well versed in evil. If nobody dies forever, then the serpent most definitely didn’t lie, because to die was not the final state. So either way, the Serpent didn’t lie when he said, “You will not die.”