Sure, he is Macomber's foil. But Wilson is also his guide – in the strictest sense of the word. He's a "professional," as he likes to tell Macomber, and that means that he will take care of stuff when things get messy or too dangerous, but he is not out to make any friends or dole out spiritual advice.
Wilson is a mentor in another way too. Because of his role as "professional hunter," he takes his client back into the field to get his buffalo. There, a transformation occurs: "He'd seen it in the war work the same way. More of a change than any loss of virginity. Fear gone like an operation. Something else grew in its place. Main thing a man had. Made him into a man. Women knew it too. No bloody fear" (4.15). It's rough going, and only lasts a short while, but with Wilson's help, Macomber finally becomes a man.