It depends: -doing it on the same computer is pretty easy.
-Doing it online with mostly client side programming requires a little programming and basic network knowledge but is not too hard.
-Doing it with mostly server side coding and using C2 mostly for display is the hardest and more network intensive but also the most secure(really hard to cheat).
Most of the time you won't need the third option, unless you're creating an MMO or any game where online is the most important aspect.
If your friend knows what he is doing, he can really implement both without it being too hard.