Romeo and Friar Laurence are foils of each other in Act III, Scene iii in two ways. The first way is their view on Romeo's banishment. The Friar thinks Romeo should be thankful that he was not killed by Tybalt instead or that the Prince didn't sentence him to death. At least with banishment, he is still alive. However, Romeo does not feel this way. He feels banishment is actually worse than death because he can't be with his beloved Juliet anymore. Romeo would rather be dead than live a life without her.
Another way in which they are foils in Act III, Scene iii is their reaction to Romeo's banishment. The Friar is trying to be the calm one in the situation, since Romeo is extremely upset. The Friar is trying to be the voice of reason and convince Romeo that banishment is so much better than what could've happened to him as a result of killing Tybalt. However, since Romeo sees things differently, he is behaving in an unreasonable, irrational, and mad fashion. He is crying uncontrollably, threatening to kill himself, and even attempts to kill himself in front of the Friar.