Nothing
He’s not lying

But if nothing happens when he says it then that mean he’s lying, therefore his nose grows.

The world would end

Time can't handle the sentence he said therefore either a black hole is created on the surface of the Earth or the universe collapses in on itself

He would fall in a coma for 1 month

Just like dividing a number by zero, ends the universe by reversing time.
Or just reversing time to the time before he said that.