Rather than seeing the friar as a foil to anybody, I think he is more strongly juxtaposed to the nurse. Both serve as confidants, and both are catalysts for the marriage of the young'ens. Both of them seem to have an unconditional love for the happiness of the lovers, but both seem to conveniently check common sense and rationality at the door when it comes to their well being.
The closest the friar comes to being a true a foil to Romeo is during Romeo's rant and tantrum at Friar's place in Act III. Romeo points out the the Friar, because of his age and membership of the clergy, could not possible understand what Romeo is going through at that point. However, this certainly doesn't stop the Friar from giving advice and pushing plans onto the two lovers.
First, let's give a brief review of what the term "foil" means in literature. A "foil" is a character who opposite in attitude and actions than that of another character.
As a priest, Friar Lawrence should have been Romeo and Juliet's foil. The two worldy immature, naive, trusting, hormonoal teenagers should have been able to depend on this older man for moral and spiritual guidance. Instead, the friar kowtows to their foolish plans, dabbles in the occult (a huge no-no), disrespects their parents wishes, and far from being the "shepherd" of his flock, Laruence leads his lambs to slaughter. He should have known better; he should have done better.
Here are a few lines from Act 3, Scene 4, in which the Friar gives the audience a foreshadowing of his dangerous attitudes. Juliet has been lamenting her pending marriage to Paris. The friar replies:
Hold, daughter: I do spy a kind of hope,
Which craves as desperate an execution.
As that is desperate which we would prevent.
If, rather than to marry County Paris,
Thou hast the strength of will to slay thyself,
Then is it likely thou wilt undertake
A thing like death to chide away this shame,
That copest with death himself to scape from it:
And, if thou darest, I'll give thee remedy.
Think about the implications of even jesting about committing a mortal sin to a teenager, especially if you are a priest!
Despite his foolish choice to aid Romeo and Juliet, Friar Lawrence is a character of reason and judgement in this story. He does not get carried away with emotion and he always preaches caution. One of his most famous quotes, from Act II, is "Wisely and slowly. They stumble that run fast." He does act quickly in marrying the lovers, but his reason for doing so is to end the feud between the families. When Romeo is exiled, he is quick to suggest a plan to get Juliet to her husband. He is the example of logic in the play.
Romeo, however, is the example of emotion. He gets caught up in his emotions, and rushes forward with them. As the play opens, he is despondent over Rosaline's rejection, and says he can not be happy again. He meets Juliet and he is over the moon. He rushes to marry her. When Mercutio dies, he rushes to attack Tybalt. When Juliet's "death" is announced, he rushes to kill himself. He does not stop to consider consequences.