In "A Narrow Fellow in the Grass", why does Emily Dickinson refer to the snake as a friend, but describe it as a threat to the author?
I do not know that Emily Dickinson in "A Narrow Fellow in the Grass" does refer to the snake as a friend. You may have read "boy" as different from the poet, as well. ("Yet when a boy...I" equates the male with the poet.) In the second stanza, Dickinson assumes the role of a boy, who would be more likely to grab at something that dazzles him. So, the poem is written throughout from the point of view of a man.
As a boy, the poet writes, he perceived the snake as a "whiplash/Unbraiding in the sun." These words connote that the boy was deceived by the snake and thought it was something dazzling and unique; the images are both aural and visual. While the sound and sight of the snake captivated the boy and dispelled any fear, whether these images evoked friendship is questionable, however.
In lines 17-20, the poet states a "cordiality" with "Several of nature's people," but in the last stanza "he" states that "he" has never come across him [the snake] without fear: "Without a tighter breathing,/And zero at the bone."