Somebody wants me to teach them Magic, but all I have are cube cards. I will not acquire new cards. Anybody have advice?
Make a simple deck that puts creatures sideways to win.
Between everything being "fair" and every type of card being encompassed Cube is the perfect venue for teaching MTG. As I tutor more and more, I become more firmly convinced that "starting with Legacy" is a far better way to teach friends than "starting with Duels," so to speak, and though the latter is a better marketing strategy, who cares, it's your Cube and your friends, after all -- that's why we're here.
Edit: if it doesn't work have him buy some Duel Decks. I typed "him" but your ambiguously gendered pronoun makes me suspicious this is a girlfriend