Robin Williams as the Genie in Aladdin sang, "You ain't never had a friend like me." And while everyone would like a Genie for a friend, that's a little extraordinary for a book called Ordinary People.
Joe Lazenby is a classic ordinary friend, and that's exactly what Conrad needs. Lazenby isn't a jerk. He supports Conrad after a fight. And most importantly, he tells him that he misses him and actually wants to be his friend. Conrad, who guards his own emotions, needs to see others be open like this.
However, Lazenby can only take so much. When Conrad tells him, "It hurts too much to be around you" (22.51)—Lazenby apparently reminds Conrad of his dead brother Buck—Lazenby gives him his distance. Their friendship is over, but not forever. It's just on a break until Conrad makes the first step back.
When Conrad learns not to avoid his feelings, even his grief over his brother's death, he realizes that being reminded of his brother isn't a bad thing. He makes peace with Lazenby in the very last chapter. With Lazenby and his comparatively normal family back in the picture, Conrad has a stable foundation to continue healing. No magic lamp required.