What weakness in Romeo does the Friar point out before agreeing to help?act 2
When Romeo comes to see Friar Lawrence, the friar points out that there are two reasons he's there so early: one, he has something really bothering him; or two, he has been out all night. After the friar chides Romeo for that, he assumes that Romeo has been out with Rosaline, and Romeo tells him he was with someone else. This fickleness, this unfaithfulness, this going from one girl (whom he has been absolutely madly depressed over) to the next is the flaw that Friar Lawrence admonishes Romeo about.
He tells Romeo that he falls in love too easily. If you remember when Romeo and his friends crashed the party at the Capulets, he had just that day pledged his love to the lovely Rosalind even though she had no use for him. Yet he glimpses Juliet at the party, and he immediately falls in "love" with her. The friar recognizes this emotional weakness in Romeo, but agrees to help him just the same.