No, he just needs to reload his materials. It wont immediately solve the problem because he'll be running the original version of the map still, and simply loading the map up again will load the map from the previous memory cache (without the compiled cubemaps).
Either type in "reloadallmaterials" with sv_cheats set to 1, or load another map (anymap will do), and then reload the map you're working on. This will then load a fresh version of your map and show your cubemap reflections (replacing your checkers).