I'm not sure if how I'm going to prove this to be correct:
Since $z$ is an $n$th root of unity, it means $z^n = 1$
For $1/z$ to be an nth root of unity, lets take it to the power of $n$,
$(1/z)^n$, and so $1/(z^n) = 1/1 = 1$, hence, $1/z$ is an $n$th root of unity ?
Nicely done! That's exactly the most straightforward way to prove it.