Chemical substitution is an effective method for tailoring the electronic and magnetic properties of functional materials. In this work, we employ magneto-optical spectroscopy to investigate the behavior of the cycloid -> homogenous ordering transition in Bi1-xNdxFeO3. From the magneto-optical response, we construct a B-T-x phase diagram that shows how the critical field decreases with Nd3+ substitution, an effect that we model with Ginzburg-Landau free energy and harmonic cycloid approximations. Compared with nonmagnetic impurities like La3+, Nd3+ is a stronger quencher and yields nonlinear B-C-x behavior. Spiral magnetic order is quenched when x similar to 0.2. With increasing temperature (T < 90 K), the critical field rises gently, a consequence of magnetic anisotropy changes with temperature. These findings extend our understanding of charge-spin coupling and advance the development of magnetic ferroelectrics.