Aims. We present a pilot study on non-local thermodynamic equilibrium (NLTE) line-formation computations for the isotopes 3 He and 4 He in the mercury-manganese star κ Cancri. The impact of NLTE effects on the determination of isotopic abundances and the vertical stratification of helium in the atmosphere is investigated. Methods. Modern NLTE line-formation computations were employed to analyse a high-resolution and high signal-to-noise ratio ESO-VLT/UVES spectrum of κ Cnc. The atmospheric parameters were determined from fitting the hydrogen Balmer lines and the spectral energy distribution. Multiple He i lines were investigated, including He i λ4921 Å and λ6678 Å, which show the widest isotopic splits.Results. Half of the observed He i lines in the spectrum of κ Cnc show significant NLTE strengthening, the effects are strongest in the red lines He i λ5875 Å and He i λ6678 Å. NLTE abundances from individual He i lines are up to a factor of ∼3 lower than LTE values. Helium is found to be stratified in the atmosphere of κ Cnc. While the LTE analysis indicates a step-like profile of the helium abundance, a gradual decrease with height is indicated by the NLTE analysis. A 3 He/ 4 He ratio of ∼0.25-0.30 is found. With the available data it cannot be decided whether the two isotopes follow the same stratification profile, or not. Conclusions. This work implies that NLTE effects may be ubiquitous in the atmospheres of HgMn stars and may have a significant impact on abundance determinations and the interpretation of the vertical abundance stratification of elements.