Proof.
By the Yoneda lemma (Categories, Lemma 4.3.5) the sheaf $\mathcal{G}_ V$ corresponding to $V \in E$ is defined up to unique isomorphism by the formula $f^{-1}\mathcal{F}(V) = \mathop{\mathrm{Mor}}\nolimits _{\mathop{\mathit{Sh}}\nolimits (\mathcal{C})}(\mathcal{G}_ V, \mathcal{F})$. Recall that $f^{-1}\mathcal{F}(V) = \mathop{\mathrm{Mor}}\nolimits _{\mathop{\mathit{Sh}}\nolimits (\mathcal{D})}(h_ V^\# , f^{-1}\mathcal{F})$. Denote $i_ V : h_ V^\# \to f^{-1}\mathcal{G}_ V$ the map corresponding to $\text{id}$ in $\mathop{\mathrm{Mor}}\nolimits (\mathcal{G}_ V, \mathcal{G}_ V)$. Functoriality in (1) implies that the bijection is given by
\[ \mathop{\mathrm{Mor}}\nolimits _{\mathop{\mathit{Sh}}\nolimits (\mathcal{C})}(\mathcal{G}_ V, \mathcal{F}) \to \mathop{\mathrm{Mor}}\nolimits _{\mathop{\mathit{Sh}}\nolimits (\mathcal{D})}(h_ V^\# , f^{-1}\mathcal{F}),\quad \varphi \mapsto f^{-1}\varphi \circ i_ V \]
For any $V_1, V_2 \in E$ there is a canonical map
\[ \mathop{\mathrm{Mor}}\nolimits _{\mathop{\mathit{Sh}}\nolimits (\mathcal{D})}(h^\# _{V_2}, h^\# _{V_1}) \to \mathop{\mathrm{Hom}}\nolimits _{\mathop{\mathit{Sh}}\nolimits (\mathcal{C})}(\mathcal{G}_{V_2}, \mathcal{G}_{V_1}),\quad \varphi \mapsto f_!(\varphi ) \]
which is characterized by $f^{-1}(f_!(\varphi )) \circ i_{V_2} = i_{V_1} \circ \varphi $. Note that $\varphi \mapsto f_!(\varphi )$ is compatible with composition; this can be seen directly from the characterization. Hence $h_ V^\# \mapsto \mathcal{G}_ V$ and $\varphi \mapsto f_!\varphi $ is a functor from the full subcategory of $\mathop{\mathit{Sh}}\nolimits (\mathcal{D})$ whose objects are the $h_ V^\# $.
Let $J$ be a set and let $J \to E$, $j \mapsto V_ j$ be a map. Then we have a functorial bijection
\[ \mathop{\mathrm{Mor}}\nolimits _{\mathop{\mathit{Sh}}\nolimits (\mathcal{C})}(\coprod \mathcal{G}_{V_ j}, \mathcal{F}) \longrightarrow \mathop{\mathrm{Mor}}\nolimits _{\mathop{\mathit{Sh}}\nolimits (\mathcal{D})}(\coprod h_{V_ j}^\# , f^{-1}\mathcal{F}) \]
using the product of the bijections above. Hence we can extend the functor $f_!$ to the full subcategory of $\mathop{\mathit{Sh}}\nolimits (\mathcal{D})$ whose objects are coproducts of $h_ V^\# $ with $V \in E$.
Given an arbitrary sheaf $\mathcal{H}$ on $\mathcal{D}$ we choose a coequalizer diagram
\[ \xymatrix{ \mathcal{H}_1 \ar@<1ex>[r] \ar@<-1ex>[r] & \mathcal{H}_0 \ar[r] & \mathcal{H} } \]
where $\mathcal{H}_ i = \coprod h_{V_{i, j}}^\# $ is a coproduct with $V_{i, j} \in E$. This is possible by assumption (2), see Lemma 7.12.5 (for those worried about set theoretical issues, note that the construction given in Lemma 7.12.5 is canonical). Define $f_!(\mathcal{H})$ to be the sheaf on $\mathcal{C}$ which makes
\[ \xymatrix{ f_!\mathcal{H}_1 \ar@<1ex>[r] \ar@<-1ex>[r] & f_!\mathcal{H}_0 \ar[r] & f_!\mathcal{H} } \]
a coequalizer diagram. Then
\begin{align*} \mathop{\mathrm{Mor}}\nolimits (f_!\mathcal{H}, \mathcal{F}) & = \text{Equalizer}( \xymatrix{ \mathop{\mathrm{Mor}}\nolimits (f_!\mathcal{H}_0, \mathcal{F}) \ar@<1ex>[r] \ar@<-1ex>[r] & \mathop{\mathrm{Mor}}\nolimits (f_!\mathcal{H}_1, \mathcal{F}) } ) \\ & = \text{Equalizer}( \xymatrix{ \mathop{\mathrm{Mor}}\nolimits (\mathcal{H}_0, f^{-1}\mathcal{F}) \ar@<1ex>[r] \ar@<-1ex>[r] & \mathop{\mathrm{Mor}}\nolimits (\mathcal{H}_1, f^{-1}\mathcal{F}) } ) \\ & = \mathop{\mathrm{Hom}}\nolimits (\mathcal{H}, f^{-1}\mathcal{F}) \end{align*}
Hence we see that we can extend $f_!$ to the whole category of sheaves on $\mathcal{D}$.
$\square$
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