what secret does the Friar and Juliet share that Paris doesn't know?
Juliet frantically visits Friar Lawrence in an attempt to get out of her marriage to Paris. Since he is one of the only people that knows that Juliet and Romeo are married, she seeks his help in avoiding this second marriage, which to a Catholic would be sinful.
Juliet is in love with Romeo and has no desire to be with Paris. Moreover, she is already married so cannot actually marry again. She fears that disclosing the information of her first marriage to her family would yield disastrous results.
In Act 4 we encounter Paris making arrangements with Friar Lawrence for the wedding which is to take place within a couple of days. Juliet runs in while Paris is there and has brief exchange with him before she able to speak privately to Friar Lawrence. During this exchange Friar Lawrence knows that Juliet is already married and he is every bit as worried for the fate of Romeo and Juliet's marriage, as well as his own if he were to perform a second marriage for her.
Paris: Happily met, my lady and my wife.
Juliet: That may be, sir, when I may be a wife.
Paris: That "may be" must be, love, on Thursday next.
Juliet: What must be shall be.
Friar Lawrence: That's a certain text. (IV.1.18-22)
Juliet, of course, avoids giving Paris any straightforward answers, but still engages in the conversation without telling him anything that is false. Friar Lawrence's comment at the end is in reply to Juliet's saying that what must be will occur. He knows the real situation and is of course certain of the disastrous consequences that will follow if Juliet goes through with the wedding, or if she discloses her first marriage.