I think FFIX would be a good start. It's the pinnacle of the time spent on a game where it's aesthetically pleasing, has good writing and world setting, and reflects both the traditional story of the older Final Fantasies while keeping in mind the trends of the newer, as per FFVII and FFVIII. It's also available as a portable game, if your friend has a PSP and really to get into the older final fantasies is to readjust your expectations in being immersed in a game. Especially when the bulk of the series are on the older generations and not so much the newer ones.

I also think that FFIV is a good starter, at least for introducing to the older series. It has a simple and clean plot, but intriguing enough to have interest. The DS version, I think, has the best gameplay of all its adaptations, and I think that it's a better experience. I think it's a good way to build a foundation of what you'd come to expect from the FF series, as it doesn't really have a complicated plot, is traditional enough to get a grasp on fantasy shorthands while also unique enough to have a bit of spin on it.

Now, for a sitting down in front of a tv kind of gameplay... I would recommend FFXII because of its setting and good sense of plot, but I wonder if it's also a bit overwhelming? It has a biiiiig world to explore, and it's a lot of dungeons. Well, I personally think it has a lot of dungeons. But I also think it's a great experience on the PS2, graphics and acting and writing-wise.