Why is this map from Leray spectral sequence zero?
I believe Tag 0BUT implies something stronger, namely that $R^{1}f_{\ast}\mathcal{O}_{X}^{\ast} = 0$. One can see this as follows: Let $\mathcal{F}$ be the presheaf on $Y$ given by $V \mapsto H^{1}(f^{-1}(V),\mathcal{O}_{X}^{\ast})$. Then $R^{1}f_{\ast}\mathcal{O}_{X}^{\ast}$ is the sheafification of $\mathcal{F}$. By Tag 0BUT, for any open subset $V \subseteq Y$ and $s \in \Gamma(V,\mathcal{F})$, there is an open cover $V = \bigcup_{j \in J} V_{j}$ such that $s|_{V_{j}} = 0$ for each $j \in J$. This means that the universal separated presheaf associated to $\mathcal{F}$ is $0$, so the sheafification is $0$ as well.
Edit : I assumed that $\mathcal O^*_X$ is quasi-coherent as $\mathcal O_X$-module which is wrong. However I think the argument should still holds since proper base change and Grothendieck theorem are valid in greater generality (i.e for sheaves of abelian groups).
If you can use proper base change, then for any sheaf $F \in \mathcal O_X \text{-mod}$ you obtain $(R^1f_*F)_y \cong H^1(f^{-1}(y), F_{|f^{-1}(y)})$ where if $i : Z \to X$ is the inclusion of a closed subset then $F_{|Z}$ means $i^* F$. Since $f$ is finite $\dim f^{-1}(y) = 0$ and so by Grothendieck's vanishing (see here) we obtain $H^1(f^{-1}(y), F_{|f^{-1}(y)}) = 0$.