"The Fly" presents Mr. Woodifield as the boss's foil, since Woodifield's relatively bumbling and feeble nature invites readers to compare him to the boss's stronger, more authoritative demeanor. In fact, even the boss himself likes to make this comparison, enjoying Woodifield's company precisely because his old friend makes him feel good about himself. By presenting Woodifield as the boss's foil, then, the story ultimately calls attention to the boss's vanity.
Take, for instance, the pride that the boss takes in showing off his fancy office to Woodifield:
As a matter of fact he was proud of his room; he liked to have it admired, especially by old Woodifield. It gave him a feeling of deep, solid satisfaction to be planted there in the midst of it in full view of that frail old figure in the muffler.
This passage makes it quite clear that the boss defines himself in opposition to Woodifield, using his old friend's "frail[ty]" as a way of feeling good about his own comparative strength and vigor—despite the fact that he's actually five years older than Woodifield. Interestingly, this section also suggests that the boss actively cares about what Woodifield thinks, since he specifically likes to have his office "admired" by "old Woodifield." This suggests that he wants his friend to be jealous of him, as if this will only add to his own sense of importance.
Of course, part of the boss's vain wish to seem younger, stronger, and more capable than Woodifield comes from the fact that both men lost their sons in World War I. Whereas this hardship has perhaps weakened Woodifield, the boss wants to think of himself as someone who has continued to thrive in the aftermath of his son's death. For this reason, the boss patronizingly pities his friend when Woodifield tries and fails to remember something he wanted to say:
“Now what was it? I had it in my mind when I started out this morning.” His hands began to tremble, and patches of red showed above his beard.
Poor old chap, he’s on his last pins, thought the boss.
This entire description of Woodifield underscores the boss's view of him as weak and pitiable: he can't remember something he was thinking about just that morning, his hands "tremble," and his complexion is unhealthy. All these things lead the boss to condescendingly think of Woodifield as a "poor old chap" who's nearing the end of his ability to function in life. More importantly, the boss seems to define himself in stark contrast to Woodifield's fragility. This is why it's especially unsettling when, in the second half of the story, he loses all sense of power and self-composure after Woodifield mentions his dead son. In fact, the story even ends with the boss forgetting what he was thinking about before torturing the fly, ultimately suggesting that he's not quite as different from Woodifield as he'd like to think.
