Tile over ridgid foam insulation

I have a basement bathroom to finish out. It was roughed in but never finished. Clients want a nice tiled shower instead of the tub and surround that it was designed for. I have build custom shower pans and showers before but this one has an outside wall I am not sure how to handle. It currently has a layer of 5/8ths sheet rock over two layers of (1.5) inch extruded polystyrene that is glued to the cement block walls. There are no fasners in this wall its all held togeter with adhesive.
There is not enough room to frame this wall so I need to know if there is a backer board that can be glued to the foam.
Also I am not quite sure how to handle fastening the shower pan liner to the foam.

Remove the drywall and foam, frame a wall, insulate it and start fresh.