Why do you think Shakespeare didn't say what happened to the Nurse and Friar in "Romeo and Juliet"?
In Shakespeare's "Romeo and Juliet," the nurse and the friar are stock characters, and, as such, are stereotypical characters who are usually not developed. As such minor characters, they serve a single function and do not usually contribute to the major theme of the literary work. Therefore, they are neither protagonists or antagonists and are not involved in the denouement of the plot, either.
The primary purpose of the Nurse is to provide comic relief. For example, in Act III, she appears with excessive clothing, billowing like a sail that must be controlled by her servant, Peter. Mercutio jokes about her and the others laugh. This comedic relief comes before the tragic death of Mercutio and the misfortune of Romeo who is banished from Verona.
The friar, on the other hand, does serve more of a purpose in the play. For, it is his interference and blunders which move the play to its tragic end. As a stock character, however, Shakespeare employs his Friar to represent the corruption of the Catholic Church, whose priests went outside their vows to delve into potions (close to witchcraft) and become too involved in the secular life from which they have removed themselves in their vows. Thus, Friar Lawrence sins in his deceptive acts of giving Juliet the potion and his close involvement with Juliet and Romeo. Such acts are outside those dictated by the Church. When confronted at the tomb, he shows no fortitude or honesty; instead he runs away: "I can no longer stay!"
The lack of complete resolution in the story line leaves the reader a margin to imagine possible outcomes for the Friar and the Nurse, based on their character profiles firmly established early on in the story. If the reader (or rather spectator!) does not learn what exactly happens to them, the Friar is not held responsible for Romeo's and Juliet's death, the blame rather being put upon the feuding families instead.
As endearing as these two characters are, the respective fates of the Nurse and the Friar are not the principle preoccupation of the story. Nor is the destiny of any other person playing a secondary role except for Romeos father, shown as seeking reconciliation between the two families at the very end of the play. The message of this tragedy is thus "enobled" by his desire to erect a golden statue in the image of Juliet.
Remember that secondary roles in Shakespeare's plays are often humorously depicted for comic relief, and any interference of that kind after the lovers' double suicide would have evidently been out of place.