Some of the more obvious ones:
Mass Effect (1 & 2)
Two of the best games this generation, period. The games break the mold of what is traditionally considered an RPG by having a hearty dose of action-oriented third person shooter gameplay (particularly ME2), but it retains some of the best storytelling in gaming. The games allows the player to greatly affect the storyline by having their decisions have a noticable effect on the environment, right up to the point you're allowed to import your character from ME1 into ME2 with ALL your decisions intact. Each game as a stand-alone is superb, but together they are more than the sum of their parts (really, get both); with ME3 on the horizon, things are only looking better.
Dragon Age
Unfortunately, the PC version for Origins (and it's expansion, Awakening) are almost universally considered superior to the console ports (much better fit with the control scheme). Nevertheless, the storyline in DA:O is very good... and REALLY long (40-60+ hours for a single playthrough, and you can greatly effect it much like ME1&2 above). The version to go for here is the "Ultimate Edition", which contains ALL the DLC and the expansion. Then we have the less well-received Dragon Age II, something which is actually a solid game (aside from a severe case of cut & paste environments) that was faced with monumental expectations. DA2
Bethedsa's Sandbox Titles
This is referring to The Elder Scrolls: Oblivion (try to get the GotY edition, contains the expansion and other DLC), Fallout 3, and Fallout: New Vegas (I'm pretty sure the last two are Bethedsa). I've only played Oblivion, but I'm fairly sure the other two are fairly similar in concept. The basis of these games is quite literally "do anything you want", and the game worlds are quite expansive. Unfortunately, like DA:O, the PC version is the way to go again. However, this time it's because of the mods... countless mods turn the mildly interesting base game into your game.
Fable II & III
A series known for over-hyping itself, the Fable series is still very enjoyable. Probably best described as a more story-driven variant of a sandbox game, you're allowed to do just about anything... though you might have to advance the central storyline to unlock more of the environment. Unlike Bioware's titles, importing your save data isn't an option and each game has it's own self-contained story... however, the story of each game is still linked in a single plotline (and it uses just enough vagueness to avoid the plot contradicting itself most of the time). I'd suggest Fable II over the third one, mostly because it is (or at least feels) a bit larger (the main storyline certainly seems longer).
The unfortunate issue is that your friend might have already played one or all of these games. If he hasn't, they're a great place to start; Mass Effect in particular seems to be right up his alley.
