Friar Lawrence, the trusted adult in this conversation, has just spent quite a few lines questioning Romeo's motives in marrying Juliet when the good friar himself has noted Romeo's pining and tears over Rosaline in recent days. Does it make sense, then, to leap to marriage immediately after meeting Juliet? No, and Friar Lawrence could have given Romeo counsel here. He could have encouraged Romeo to slow down and not rush to emotional action. Even more compelling is the friar's reasoning for agreeing to this marriage: To turn the Capulet and Montague feud into love. He is, therefore, treating Romeo and Juliet like pawns.
The lines that showcase the guilt of Friar Lawrence in the deaths of Romeo and Juliet are found in act 4, scene 1:
FRIAR LAWRENCE: 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.
An if thou darest, I’ll give thee remedy
It's important to remember that Juliet is not yet 14 in this play. When Juliet comes to him for help in avoiding a marriage with Paris, Friar Lawrence offers a poison to this child that is so deadly that she must "copest with death himself to 'scape from it." It is daring and risky and not a responsible option for an adult to provide to a young person as a solution to her problems.
In act 5, scene 2, Friar Lawrence talks with the friar whom he has tasked with delivering the letter of crucial importance to Romeo:
FRIAR LAWRENCE: Unhappy fortune! By my brotherhood,
The letter was not nice but full of charge,
Of dear import, and the neglecting it
May do much danger.
In this letter to Romeo, Friar Lawrence explains that Juliet is faking her own death in order to join Romeo in his exile. He entrusts this critical information to Friar John, not even telling him that the letter could be of life-or-death importance. Friar John takes the task casually and brushes off his inability to deliver it. Friar Lawrence clearly should have made the importance of the letter a bit more clear or should have delivered the letter himself, thus ensuring Romeo received the information.
Throughout the play, there is certainly compelling evidence that points to Friar Lawrence's guilt in the deaths of Romeo and Juliet.