In mathematics, we use non différentiable for non-differentiable functions just like we use non continue for non-continuous functions.

Indifférentiable has no definite mathematical meaning, it’s only used for “impossible de trouver une différence” (very close to indistingable which means similarly “impossible à distinguer”), and it might apply to functions as well. “Deux fonctions non différentiables” and “deux fonctions indifférentiables” have different meanings.