Maybe you should have shown the client Here Maps instead, which is available on all WP8 and Windows 8 machines. Much, much better than Google Maps, since you can download maps and don't need a data connection.
From a hardware perspective, the Nexus 5 is an upgrade from the 8X in some spots - faster processor, bigger screen, more RAM. Otherwise, the camera isn't that great of an improvement, it is heavier than the 8X and the OS is still the same.
Cannot tell you how to spend the cash, but perhaps you're better off waiting to upgrade...