Recently, quadrupole helix chirality and its domain structure was observed in resonant x-ray diffraction experiments on DyFe_3(BO_3)_4 using circularly polarized x rays [T. Usui et al., Nature Materials 13, 611 (2014)]. We show that this effect can be explained quantitatively by calculating the quadrupole moments of the Dy^3+ ions induced by the low-symmetry (C_2) crystal-field (CF) component. In this work, the CF parameters for the Dy^3+ ions in the P3_121 (P3_221) phase of DyFe_3(BO_3)_4 are

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... btained from CF calculations based on the analysis of high-resolution temperature-dependent optical spectroscopy data. We also consider the helix chirality of the single-site magnetic susceptibility tensors of the Dy^3+ ions in the paramagnetic P3_121 (P3_221) phase and suggest a neutron diffraction experiment to reveal it.