'God' has never told us (the human race in it's entirety) anything about right or wrong. This information has always come from humans in whichever form that may be in so that's that out of the way.
The glib statement from Mr lewis says nothing because he himself did not try to describe what a 'straight road' is. Had he tried he would possibly have realised that there is no such thing, zoom in enough and all roads are crooked. A truly straight road can only exist in people's minds, never in reality, which makes this analogy very good but it actually means the opposite of what he thought: In the real world there is NO absolute right, and therefore no absolute wrong, the judgement of how wrong something is depends on who is doing the judging.