Why is “Friends” capitalized in 1 Maccabees in the NRSV?
The NRSV is not the only version of Scriptures that has the term Friends of the King capitalized. Other versions do the same.
The origins of the King's Friends can be seen in the history of Hushai a friend of King David and a spy according to the Hebrew Scriptures.
Hushai or Chusai was a friend of David and a spy according to the Hebrew Bible. During Absalom's rebellion, as described in the Second Book of Samuel, he agrees to act as an advisor to Absalom to sabotage his plans while secretly sending information to David. It was on his advice that Absalom did not immediately pursue the retreating David, thus giving David time to regroup and gather his forces. Hushai's advice helped to ensure Absalom's rapid defeat.
Hushai was an Archite, that is, a native of Archi, a place south of the portion of Ephraim, near Bethel (Joshua 16:2). He is called in 1 Chronicles 27:33 "the king's friend". This title is similar to that of counselor given to Ahitophel, or that of leader of the army to Joab: there is a similar use of the term in 1 Kings 4:5. In the Books of the Maccabees it is an official title given by the Seleucids to persons of confidence who have important military or civil functions (1 Maccabees 2:18; 3:38; 6:10, 14, 28; 7:6-8; etc.). It is likely then, that Hushai's title of "friend" of King David does not imply the intimate relations suggested by the term.
Nevertheless, the account that is given of him during Absalom's rebellion (2 Samuel 15 - 17) shows that in his case the title was not merely official. After David has heard of the treason of his counselor Ahitophel, he organizes an evacuation of Jerusalem. On his way up the Mount of Olives he is met by Hushai, whose garments are rent and whose head is covered with dust. He is probably an old man, for David tells him he would be a burden in the flight.3 But the king does honour to Hushai's cleverness by sending him to Jerusalem, to "defeat the counsel of Ahitophel". Hushai persuades Absalom to take him into his confidence, and, in the council held shortly afterwards in regard to the measures to be taken against David, he overrules Ahitophel and obtains a delay which secures the safety of the king. He is able likewise to convey information to David through Zadok, Abiathar, and their sons: Ahimaaz, Zadok's son and Jonathan, Abiathar's son. (2 Samuel 15:36) - Hushai (Wikipedia)
The Jewish Virtual Library states that Hushai actually held the office of the King's Friend and as such it would be capitalized:
HUSHAI THE ARCHITE (Heb. חוּשַׁי הָאַרְכִּי), biblical figure listed in I Chronicles 27:33 as holding the office of "the king's friend" under David. In II Samuel 15:37; 16:17 he is referred to as "David's friend." Hushai figures prominently in the story of the rebellion of *Absalom. At the time of David's flight from Jerusalem, Hushai, deeply grieved and wearing the traditional rent garments and ashes, sought to join David's company on The Mount of Olives, to which they had fled when Jerusalem's capitulation to Absalom appeared inevitable. David, however, persuaded Hushai to return and offer his allegiance to Absalom, so that he might defeat the counsels of *Ahithophel, David's adviser, and that he might supply information to David (II Sam. 15:32–37). Hushai, accepted as a loyal adviser by Absalom, successfully opposed Ahithophel's plan to pursue and attack David immediately, proposing instead that Absalom mass his forces and attack David in person. Having thus afforded David time to escape, Hushai sent word to David through his couriers, the sons of the priests *Abiathar and *Zadok, to cross the Jordan immediately (II Sam. 17:5–16). Although no more is heard of Hushai himself, Baana son of Hushai, one of the prefects of Solomon listed in I Kings 4:16, is probably his son.
The term "the Archite" indicates that Hushai belongs to the clan named in Joshua 16:2–3 as dwelling in the vicinity of Ataroth, on the border between Ephraim and Benjamin. The name Hushai itself is most probably a short form of the name Ahishai, Ahushai.