What role did Friar Lawrence play in uniting Romeo and Juliet? Was he successful?
If you were to consider Romeo and Juliet as united in death, the afterlife, I would have to say he was successful. However, the Friar far oversteps his bounds, especially as a man of God, when he meddles in Romeo's love affairs. He questions Romeo in Act II, Scene 3: "Was thou with Rosaline?" It is as if the Friar, in his bumbling way, is living vicariously through the teenager's life. In this same scene, Romeo confesses he no longer loves Rosaline, it is now a Capulet, Juliet, who hold his heart. The Friar, without question, immediately becomes part of their secret affair:"I'll thy assistant be" (2.3.90).
Later, as we know, the Friar uses his holy power to secretly marry the two lovers in hopes of joining the feuding houses: "...you shall not stay alone till Holy Church incorporate two in one" (2.6.37). Unfortunately for the Friar, he is instrumental in the deaths of Romeo and Paris, and his only solution for Juliet prior to her suicide is to "Come, come away...I'll dispose of thee among a sisterhood of holy nuns" (5.3.154-157). What young girl in love, with her lover dead before her, would choose to go to a nunnery? Thus, the Friar may be motivated by good intent, yet by meddling in the teenager's lives, he plays a large role in the death of the innocents!
Friar Lawrence is Juliet's confessor. The two go to him to tell them of their love, and he agrees to secretly marry them in the hope that this marriage will stamp out the feud that their families--the Montagues and the Capulets--have been engaged in for so long.
As far as working, yes, Romeo and Juliet are married. Romeo does sneak into Juliet's room before his banishment and consumate the marriage. However, they are separated by Romeo's banishment and also by Juliet's father's plans to have her marry Paris.
The Friar then comes up with a plan to reunite the two by giving Juliet a potion to make her appear to be dead while she is only sleeping. Unfortunately, Romeo did not receive the message from the Friar telling him the truth. Romeo hears only that Juliet is dead.
Romeo swallows poison, Juliet awakes and stabs herself. Now they are both truly dead.
At the play's end, however, the narrator indicates that the deaths of their only children make the Capulets and Montagues realize what their feud has cost them. So, in a weird kind of way, you can argue that Friar Lawrence was indeed successful.