Scattering by magnetic impurities is known to destroy coherence of electron motion in metals and semiconductors. We investigate the decoherence introduced by a single act of electron scattering by a magnetic impurity in a quantum Hall system. We introduce a fictitious nonunitary scattering matrix S for electrons that reproduces the exactly calculated scattering probabilities. The strength of decoherence is identified by the deviation of eigenvalues of the product SS† from unity. The degree of nonunitary is related to the phase uncertainty acquired by an electron after one scattering event, and its square is inversely proportional to the electron's coherence length. We estimate the width of the metallic region at the quantum Hall effect inter-plateau transition and its dependence on the exchange coupling strength and the degree of polarization of magnetic impurities.