According to the book of Nehemiah, the important people of Judah were friends with a man named Tobiah and they kept sending letters to each other. Even though Nehemiah was trying to protect the city, these letters made his job harder because Tobiah was not on his side.
Nehemiah 6:17: Moreover in those days the nobles of Judah sent many letters unto Tobiah, and [the letters] of Tobiah came unto them. [sent...: Heb. multiplied their letters passing to Tobiah]
Nehemiah 6:17 details alliances through letters between nobles and Tobiah, suggesting friendliness, whereas Proverbs 27:6 implies that faithful are the wounds of a friend, indicating distrust of outward friendships.
Proverbs 27:6: Faithful [are] the wounds of a friend; but the kisses of an enemy [are] deceitful. [deceitful: or, earnest, or, frequent]