How would you describe the style in which this story ("Good People" by David Foster Wallace) is written? How does the style serve the story?

Stylistically, it is notable that this story contains no actual quoted dialogue. It would be a good idea to try to figure out why the author would choose to represent dialogue between Lane Dean and Sheri only indirectly. First, this kind of indirect reporting can keep the reader at an emotional distance from the characters. It does feel, in some ways, that Lane Dean is trying to retain an emotional distance from Sheri, from the baby they accidentally conceived, and from the abortion he wants her to get. In fact, he does not love her—he knows this, and she does too—and so perhaps the author keeps the reader at an emotional distance in order to replicate Lane's sense of detachment and increase the awkwardness we feel. Often, a third-person omniscient narrator, one who can report on all characters' thoughts and feelings, is used for a similar purpose. When we know everyone's thoughts and feelings, we grow no closer to anyone.
You might also consider what the older man at a nearby picnic table could represent. He stands and seems out of place in his suit coat "and the kind of men's hat Lane's grandfather wore in photos as a young insurance man." He appears to be looking out over the lake, and he does not move. Perhaps he is symbolic of the Christian God, in which Lane and Sheri believe. He waits nearby, not interfering, to see what they will do. Maybe he is a metaphor for Lane's conscience: this could explain why he reminds Lane of his grandfather and traditional values often associated with the older generation. There are other possibilities, too. Reread the story and see what you think about the man—why is he there at all?