Among the newly discovered features of multiple stellar populations in globular clusters (GCs), the cluster NGC 1851 harbours a double subgiant branch (SGB), that can be explained in terms of two stellar generations, only slightly differing in age, the younger one having an increased total C+N+O abundance. Thanks to this difference in the chemistry, a fit can be made to the SGBes, roughly consistent with the C+N+O abundance variations already discovered two decades ago, and confirmed by recent spectroscopic data. We compute theoretical isochrones for the main sequence turnoff (TO), by adopting four chemical mixtures for the opacities and nuclear reaction rates. The standard mixture has Z = 10-3 and [α/Fe] = 0.4, the others have C+N+O, respectively, equal to 2, 3 and 5 times the standard mixture, according to the element abundance distribution described in the text. We compare tracks and isochrones, and show how the results depend on the total CNO abundance. We note that different initial CNO abundances between two clusters, otherwise similar in metallicity and age, may lead to differences in the TO morphology that can be easily attributed to an age difference. We simulate the main sequence and SGB data for NGC 1851 and show that an increase of C+N+O by a factor of ~3 best reproduces the shift between the SGBes. According to spectroscopic data by Yong et al., the C+N+O abundance in this cluster appears correlated with the abundance of s-process elements, Na and Al, and this makes massive asymptotic giant branches (AGBs) the best progenitors of the C+N+O enriched population. We compare the main sequence width in the colour mF336W-mF814W with models, and find that the maximum helium abundance compatible with the data is Y ~= 0.29. We consider the result in the framework of the formation of the second stellar generation in GCs, for the bulk of which we estimate a helium abundance of Y <~ 0.26. The precise value depends on which are the AGB masses from which the C+N+O enriched matter originates, and on the amount of dilution with the pristine gas.