How does the nurse offend Juliet in this act and cease to be her friend? How does this development add to the tragedy of the events that follow?

The nurse concedes that perhaps Paris is a better choice than Romeo for marriage. The Nurse sees the writing on the wall: if Juliet does not marry Paris, she will be disowned by her father. The Nurse promises to find Romeo – whom she knows is hiding with Friar Laurence - and bring him to Juliet's bed that night. Juliet gives the Nurse a ring for Romeo to wear when he comes to see her. This helps set up the final tragedy of the play.