The nurse becomes an accomplice of the Friar by advancing the plot. What does she do and say that does this?
When Romeo is banished after he kills Tybalt, he immediately goes to the friar, his ever-faithful mentor. Yes, the friar bears much responsibility for what happens ultimately to the young lovers.
While Romeo is at the friar's cell, Juliet is finding out about what he did from her mother. Juliet does the best she can not to let her mother know how she truly feels about Romeo. Lady Capulet reads her sorrow as grief over her cousin's death.
The Nurse acts as an accomplice when Juliet freaks out about Romeo's banishment and agrees to bring word to Romeo of her love. The Nurse brings the gift of a ring and reports that Juliet is weeping and crying and wailing just like Romeo is. In this regard, the Nurse is in true cooperation with the planned secret marriage and the furthering of their relationship.
I do not think that the Nurse is nearly as involved in the plot as Friar Laurence is, but she does play some amount of a role in moving forward the main action of the play. She does this by helping Romeo and Juliet get together and get married.
The night after the balcony scene, Juliet sends the Nurse to Romeo to find out what his intentions are. When she finds that Romeo is truly in love with Juliet, she helps by doing such things as bringing the rope ladder from Romeo to Juliet so that Romeo will be able to get up to Juliet's room.
But, on the other hand, the Nurse thinks Juliet should marry Paris even after she's married to Romeo. So it's not like she's pushing things along the way Friar Laurence does.