The nurse went against Juliet's parents' wishes and told her it was okay to go ahead and marry Romeo. She knew Mrs. Capulet arranged for Juliet to marry Paris, but she still encouraged Juliet to marry Romeo. "Then hie you hence to Friar Lawrence' cell; there stays a husband to make you a wife" (pg. 331, lines 68/69). By saying that, she over-stepped her boundaries of being just Juliet's nurse.
Friar Lawrence is to blame because he's the one that married Romeo and Juliet. The only reason he did is because he thought it would stop their family feud, which is not the right reason to marry anyone. "In one respect I'll thy assistant be; for this alliance may so happy prove to turn your households' rancor to pure love" (pg. 322, lines 90-92). By saying this, it means that he did it because of the family feud, but when someone marries two people it should be because they want to and because they truly think that those two people will last a lifetime together.
The Friar didn't anticipate all of the negative things that could happen when he faked Juliet's death. He didn't think of how the Capulets would react to losing their only daughter and how much pain he would put them through. "Shall Romeo by my letters know our drift; and hither shall he come; and he and I will watch thy waking, and that very night shall Romeo bear thee hence to Mantua" (pg. 363/364, lines 114-117). He only had so little time to fake Juliet's death before she had to marry Paris, so his plan was not very well thought-out at all. He also didn't anticipate someone else seeing Juliet lying in her casket and