Your friend sounds like someone I would greatly disagree with.
Multiplayer is great, but not every game needs it. Many games have been worse off because developers have forced multiplayer into it. My opinion may be a bit skewed, but I'm the kind of person that prefers a single player experience to multiplayer(not that I don't enjoy it). No offense to your friend (well maybe a little) but he sounds like the type of gamer that plays nothing but FPS's and when given a game that's not MWF or CoD gets a bit of a "Wtf?" look on his face when confronted with actual story. In short, your friend experiences games on a very shallow level and I weep for him.
For me games are an intimate experience, I treat them much like I treat movies, books, and television shows. To me a good game has sound cinematic, gameplay, story, and art direction. Multiplayer comes as a bonus for when I have friends over and I want to show off the game but not bore them by playing it alone.
Does a modern game need multiplayer to be good? Of course not, we need look no further than the God of War series, Psychonauts, Prototype/Infamous, or Assassin's Creed to be shown that the truely great games have made their claim not in multiplayer but in the intimate single-player experience that allows for the conveyance of an amazing visual and interactive experience.
-EDIT-
This is not to say that games such as the Left 4 Dead series do not also hold a spot amongst the great games(I'm a bit sleep deprived but what I mean is multiplayer games can be legendary as well) Indeed I have found it to just as enjoyable as my single-player games because of it presentation of the multiplayer.