Your tutor is wrong. Every homeomorphism (such as $\exp:\mathbb R\to\mathbb R_{>0}$) is closed. The standard topology on the codomainset $\mathbb R_{>0}$ is thea closed set of all open sets of $\mathbb R$ intersected with $\mathbb R_{>0}$, and in this topology, the setstandard topology on $\mathbb R_{>0}$ is a closed set.