If your friend is a gamer already (enough to be familiar with a variety of modern or classic games), here are some of what I consider the key highlights of DF:
* DF is a non-linear, open-world, "sandbox"; to a far greater degree than most games that use those adjectives. You could even argue that (for some people at least) it's more of a "software toy" than a "game" in the sense of the old Maxis SimWhatever definition (SimEarth and SimAnt in particular, amusingly enough).
* DF features an radically high level of procedurally generated content, and it's growing.
* The combination of the first two features above means that it has an unusually high replay value, to the extent that multiple fortresses can feel like entirely different games. *Most* of the coolest stuff that happens in DF is the result of "emergent behavior" at multiple levels.
* The combination of the first two features above means that the game is very much influenced by what the player brings to the table. The fortresses and projects a player designs arguably says more about the player than the game.
* The combination of the first two features above means that there is very little hand-holding, and the difficulty adjustments are low-res and infrequent. There are no guarantees that a game start has what you want in convenient places, or at all. You can be playing along for hours and think you're doing OK, and then suddenly you get some unexpected threat, there's a brief interlude of blood and horror that escalates crazily out of control, and then you're staring at the "Your fortress has crumbled to its end" screen.
* You cannot win DF. You can *loose* DF, however, and most players will do so *many* times. The somewhat default experience assumption is that progression is intended to be like classic Roguelikes with perma-death and only one save; each trip into the dungeon, you learn a bit more of the skills and special knowledge necessary to push your inevitable demise a bit further off in your next play-through. People who are not comfortable with the idea of loosing over and over as one advances will likely feel stressed by DF, and probably want one of the "cheat" editors, or to make lots of backup save games to allow "going back in time".
Much like real life, in the end the only guarantee is a tombstone; what matters is the epitaph on it and the stories you've left with friends along the way. This ties directly to the DF mantra, "Loosing is Fun!".
* DF is both more early-access and more indy-developer than most games that use either label. Some people love this, some tolerate it, some find it annoying.
* DF is highly moddable, requiring no special software to enact major changes. There is a rich community that supports a wide spectrum of mods from common utilities to highly obscure total conversions. Mods are available to make it more "realistic" (at the possible expense of gameplay) and more "game-like" (at the possible expense of realism).
* The default arrangement of DF is a considerable dose of detailed realism in places where most games make simplifying assumptions, interspersed with a bizarre spicing of unexpected simulation edge cases and places where fantasy world assumptions go to die horribly.
* In the end, you have seven dwarves, a few tools and basic supplies, and an untapped wilderness stretching out in front of you. What do you build?