Stellar metallicity distribution functions (MDFs) have been measured for resolved stellar populations in the outer halos of many galaxies in nearby groups. Among them, the MDF of NGC 5128, the central giant elliptical in the Centaurus group, provides essential constraints for theories of massive galaxy formation and hierarchical assembly. To investigate the formation and chemical evolution history of the outer halo of giant elliptical galaxies, we examine the chemical properties of three zoom-in high-resolution cosmological hydrodynamical simulations of an NGC 5128-like giant elliptical galaxy and compare their outer halo MDFs to the observed one of NGC 5128. Even though the simulated galaxies have different merging histories and age distributions, all predicted MDFs are in good qualitative agreement with the observed one. The median metallicity of the simulated galaxies is, on average, [M/H] = -0.41 +/- 0.06 compared to the observed value of [M/H] = -0.38 +/- 0.02 for NGC 5128, and the dispersion in metallicity is similar to 0.77 dex for both observed and simulated galaxies. We investigate the origin of the stars ending up in the outer halo field of simulated galaxies and show that most have an "accreted" origin, formed in other small galaxies and later accreted in mergers. Only similar to 15% of the stars are formed in situ within the main progenitor of the galaxy and radially migrate outward. We show that the contribution of metal-rich in situ stars is subdominant in the outer halos of our simulated galaxies but can be prominent in the inner regions.