In this paper we prove a rigidity statement for free boundary minimal surfaces produced via min-max methods. More precisely, we prove that for any Riemannian metric g on the $3$-ball $B$ with non-negative Ricci curvature and $\mathrm{II}_{|\partial B}\ge g_{|\partial B}$, there exists a free boundary minimal disk $\Delta$ of least area among all free boundary minimal disks in $(B, g)$. Moreover, the area of any such $\Delta$ equals to the width of $(B, g)$, $\Delta$ has index one, and the length of $\partial\Delta$ is bounded from above by $2\pi$. Furthermore, the length of $\partial\Delta$ equals to $2\pi$ if and only if $(B, g)$ is isometric to the Euclidean unit ball. This is related to a rigidity result obtained by F.C. Marques and A. Neves in the closed case. The proof uses a rigidity statement concerning half-balls with non-negative Ricci curvature which is true in any dimension.