Tell him to unsubscribe from all of his addons then install the WG CWRP Content pack, once hes done that type "menu_cleanupgmas" in console and try connecting to the server. If that doesn't work a few other things he can try is typing "cl_updaterate 15" in his console and then connecting directly through the console with "connect 64.52.81.101:27015". And finally if none of that works try a fresh reinstall of Garrys Mod. (The server is currently down though so he won't be able to connect until its up either way)