Based on the information, the friar fears that Romeo may have slept in sin with Rosaline after the visit.
In Act 2, Scene 4 of Romeo and Juliet, there is a fair amount of joking around among Benvolio, Mercutio, and Romeo, but the important things that happen are that we learn that Tybalt is aggressively challenging the Montagues to a fight and Romeo and the nurse set up a plan for Romeo and Juliet to get married.
The friar proposes a plan: Juliet must consent to marry Paris; then, on the night before the wedding, she must drink a sleeping potion that will make her appear to be dead.
Juliet will be laid to rest in the Capulet tomb, and the friar will send word to Romeo in Mantua to help him retrieve her when she wakes up
After the visit, Romeo assures him that did not happen, and describes his new love for Juliet, his intent to marry her, and his desire that the friar consent to marry them that very day.
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