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!