In this paper we study the effect of a surrounding lattice of He atoms on the manifold of electronic states of the H2+ molecule-ion. One-center expansions of the molecular wavefunctions are employed, calculated by the Tibbs-Wannier method. The spherically averaged wavefunctions are in good agreement with the known exact solutions. A detailed study of the environmental effect of the He lattice leads to the prediction of a blue shift of the first electronic transition, arising from a delicate balance between changes in the impurity excitation energy, Coulomb, exchange, van der Waals, and three-center interaction terms. The signs of the various energy changes are rationalized in terms of the overlap charge density. These results are compared with previous treatments of environmental spectral shifts based on continuum models.