I'd need The One episode though! (And I don't see The Cage as being that particular ep myself! -Sure, I would've bought the box-set, but I don't think this guy would!Selected TOS episodes (remastered).
I think very highly of "The Corbomite Maneuver" myself (upon recent viewings I like it better every time I see it...I'm sorta like, "Corbomite Maneuver, where have you been all my life?"), and also respect a suggestion elsewhere in this thread of "Where No Man Has Gone Before." There is a set of three war-feel episodes in the first season that I sure do love ("Arena," "Balance of Terror," "Errand of Mercy") and it is worth noting that each needs little or no prior Trek knowledge and introduces the enemy on its own.
I really do feel that TOS Remastered gives you the best of both worlds as a way to introduce Trek to an older gentleman. He will recognize elements of the politics, production design and so forth as being from days gone by, but the show looks pretty great, especially as it might compare to the expectations of the uninitiated.
Re: the suggestion of the the DS9 episode "Duet," it is a superb piece of TV, but I do feel that coming into the whole Bajor backstory cold might dull the impact of it a little bit.