Depends on how committed they are. I could give them an entire RPG history lesson (starting with Dragon Quest 1, because my pre-DQ RPG knowledge of Ultima and Wizardry and stuff is poor) and currently ending with Divinity: Original Sin).

Or I could just pick out a few games I think are the best; or a few games that I think even a non-RPG player could appreciate, or whatever. There are a lot of ways to go. Or the most influential/iconic RPGs? I'd really have to evaluate the character of the person playing them.

On the other hand I actually kind of did this for my sister, and I made her play:
DQ1, 3, 4
FF6
CT
Earthbound
Xenosaga

Of course that was quite a while ago now so any new-ish games couldn't have been included, and she probably wouldn't have played more than about that many games.