The exchange coupling of the helical edge state of a quantum spin-Hall insulator with an easy-plane magnet has no effect on its dc electrical conductance if the magnet's anisotropy axis is aligned with the spin-quantization axis of the helical edge state [Meng et al., Phys. Rev. B 90, 205403 (2014)]. We here calculate the ac conductance GV (w) and the noise power SV (w) in the presence of a dc bias V. While both take the universal values GV (w = 0) = e2/h and SV (w = 0) = 4e2kBT /h in the zero-frequency limit, GV (w) and SV (w) are quickly suppressed for finite w, so that low-frequency transport is effectively noiseless.
