Besides the many useful suggestions below about which software to use for this assuming he wants to make a COMPUTER game, I would also explore what is his idea? Could you just "build" it with cardboard, a board drawn on a computer and printed out and various physical pieces? Or at least, do that for a prototype of the core part of the game and gameplay? I think it would be useful to learn the concept of a "minimum viable product" in this context (or rather, "minimum quantum of utility" to get away from preconceived ideas about what a "product" is..). Being able to get it to a point where people can first actually try to play it is really useful, and lets you get feedback early without all the overhead of the actual computer programming (if needed)