I agree, but would also like to add a bit more to the previous answer. The Friar bears a good deal of the blame because he *should* have been the most reliable. Juliet has essentially no adult to turn to for sound advice in the play. Her father has already proven himself rash, her mother distant, and her nurse loving but incompetent. The only other adult she can turn to is the Friar. He is the principle adult who could have steered the young couple in a proper direction.
At first, the Friar seems like someone who should be trusted. He tries to warn Romeo of the temporary state of infatuation: “These violent states have violent ends / And in their triumph die, like fire and powder, / Which as they kiss consume… / Therefore love moderately; long love doth so. Too swift arrives as tardy too slow” (2.6.9-11, 14-15). But his words carry no weight, because his actions undermine them. He marries the couple anyway, despite the fact that he knows he is violating parental wishes, as well as wedding two people who have continually demonstrated to him a blatant disregard of reason.
Of course, the Friar’s final misdeed comes when he gives Juliet the vial that will make her appear to be dead. Even if one could somehow dismiss this uncomfortable element, it is impossible to ignore the fact that Friar Laurence has been instrumental in bringing the crisis to boil. One perhaps could argue that the Friar was making a desperate bid to save her life, because Juliet was threatening to commit suicide. Unlike Juliet’s nurse, however, the Friar should have been grounded in morality and used his educated mind to reason with her instead of adopting such an untenable scheme.
He is the one of the few adults that knew the full story and could have done more about it. He could have confessed to everyone that he had married the teenagers or he could have outright refused Romeo's request for marriage.
Friar Lawrence could be blamed for the tradegy of Romeo and Juliet because Romeo came to him with the request for he to marry Romeo and Juliet that very night and he knew they would not have told anybody else. Friar's reasoning for marrying them without telling any of the parents was that he believed this marriage would end the centuies long feud between the two households.
But that's only the first offence. The second? Bringing about this potion and urging Juliet to drink it so that she could pretend to be dead. This was a good idea but there was no guarantee that he would get the idea to Romeo in time(which he didn't) and so the ending occured.
Basically, he didn't play the responsible adult to the two love struck teens like he should have.