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Current three-dimensional (3D) hydrodynamical model atmospheres together withNLTE spectrum synthesis, permit to derive reliable atomic and isotopic chemicalabundances from high-resolution stellar spectra. Not much is known about thepresence of the fragile 6Li isotope in evolved solar-metallicity RGB stars, notto mention its production in magnetically active targets like HD123351. Fromfits of the observed CFHT spectrum with synthetic line profiles based on 1D and3D model atmospheres, we seek to estimate the abundance of the 6Li isotope andto place constraints on its origin. We derive A(Li) and the 6Li/7Li isotopicratio by fitting different synthetic spectra to the Li-line region of ahigh-resolution CFHT spectrum (R=120 000, S/R=400). The synthetic spectra arecomputed with four different line lists, using in parallel 3D hydrodynamicalCO5BOLD and 1D LHD model atmospheres and treating the line formation of thelithium components in non-LTE (NLTE). We find A(Li)=1.69+/-0.11 dex and6Li/7Li=8.0+/-4.4 % in 3D-NLTE, using the line list of Mel\'endez et al.(2012), updated with new atomic data for V I, which results in the best fit ofthe lithium line profile of HD123351. Two other line lists lead to similarresults but with inferior fit qualities. Our 2-sigma detection of the 6Liisotope is the result of a careful statistical analysis and the visualinspection of each achieved fit. Since the presence of a significant amount of6Li in the atmosphere of a cool evolved star is not expected in the frameworkof standard stellar evolution theory, non-standard, external lithium productionmechanisms, possibly related to stellar activity or a recent accretion of rockymaterial, need to be invoked to explain the detection of 6Li in HD123351.

Lithium abundance and 6Li/7Li ratio in the active giant HD123351 I. A comparative analysis of 3D and 1D NLTE line-profile fits