We have $\bigwedge_{i\in I}\arrow{t_i}{s_i}\leq t$.
Using the definition of type schemes, let $t'=\bigwedge_{i\in I}\arrow{t_i}{s_i}\land\bigwedge_{j\in J}\neg\arrow{t'_j}{s'_j}$ such that $\Empty\neq t' \leq t$.
The induction hypothesis gives, for all $i\in I$, $\Gamma,x:s_i\vdash e':t_i$.