Answer: I don't believe they are acting irresponsibly because they are trying to help Romeo and Juliet be happy. They know that their parents hate each other and they think that that shouldn't stop them from being happy together
The Nurse just wants Juliet to be happy although she does tell Juliet that she would be better off with Paris. The Friar is the same way except for Romeo and he believes the marriage would end the fighting between the Montagues and the Caplets. Both the Nurse and Friar see that these two are in love with each other and they can't stay apart. If the Nurse or Friar were to tell Juliet's parents they would more than likely have Romeo arrested (he ends up getting hunted by the kings' men because he kills Tybalt in honor of Mercutio and in defense of his love for Juliet). Hope this helped!
