Tales of Vesperia. You can get over 100 hours, easy. Plus, its 100 hours of happy-meth in a vido game form.

He means replay value as in going back and playing the game multiple times not simply a long game to play.

Mass Effect
Lost Odyssey

Left4dead if you play with Friends,Maybe Fable 2 and Mass Effect for sure.

You can do that too.

Mas Effect
Tales of Vesperia
Halo 3
Fallout 3
Lost Odyessy
Fable II
GTA IV
Forza 2
Cod 4- online wise
L4D
Oblivion

Frankly twenty hours is setting the bar incredibly low. The truth is that over ninety percent of the games on the console can give you at least twenty hours. A better bar is somewhere over seventy hours. That is where you thin the heard. You probably should elaborate on what kind of games this friend of your likes to play. Otherwise members will just recommend their favorite games.

Safe bets

Halo 3 ODST. The single player for Halo 3 was decent, and not necessarily outstanding. You could say its rather on the short side. ODST will also probably be quite short in the single player. The big difference is that it comes with all the map packs. The online play is where the real value can be found it easily has hundreds of hours of play.

Grand Theft Auto 4. The beauty of the series, and the latest game is its a open world sandbox. The main story can run you around fifty hours with another ten hours if your doing side quests. Then you have the online component good for another ten hours at least. Then as we all know it is a good tension releasing game. You will pop it in months or years later to replay from start to finish, or just to tear around town running down pedestrians. Easily eighty hours of play.

Mass Effect. The game is perhaps the most accessible of the role playing games. The character development is simple, and you can play it in many ways. You can play it as a shooter, a tactical shooter, or if you play the wheel it can even be more like the online role playing games with its cool down times. Not to mention it is story driven, and there is ample incentive to replay the game again to see what the alternate possibilities were. Easily seventy hours of play.

Fallout 3. Another game that gives you more freedom. Play it as a shooter, or play it as a turn based affair. Once again dialogue gives the player incentive to play again to see what all the different outcomes are. This game is racked with content. So you can get around one hundred and twenty hours of play easily if you want to do everything. That is even before you consider buying extra content. Toss that in, and you got yourself almost to two hundred hours.

Fable 2. Perhaps the most casual friendly role playing game on the platform. It is equal parts a role playing game, world management game, and social simulation game. So there is a pretty good chance that your friend will at least be attracted to one aspect of the game. Easily seventy hours.

Bioshock. Well it is a bit more difficult, but then again it is also a little more rewarding. Everyone seems to be forgetting this game now that it is no longer a exclusive. However all the variation in play really stands the test of time. Easily fifty hours for the first play through, but you almost have to take a stab at a second play through. Which should get you to at least seventy hours.

I got 1000 GS on Bioshock and it didn't even take me 40 hours >_>

Mass Effect
Crackdown
Halo 3
Star Wars: The Complete Saga
Banjo Kazooie: Nuts and Bolts

Games that i've played:

RE5
Halo 3
Gears of War 1 & 2