What quotes show the nurse and friar are responsible for Romeo and Juliet's deaths?

No one is really malicious about causing Romeo and Juliet’s deaths. Their deaths are caused by a combination of circumstances and bad choices. You could argue both deaths were caused by Friar Laurence and the nurse keeping quiet about the secret wedding between Romeo and Juliet. They enabled Juliet and put her in a position she could not escape.

Juliet uses the nurse as a go-between between her and Romeo. The nurse agrees to do this because she cares about Juliet. She knows Juliet would be better off not getting involved in with a Montague, but plays along to make Juliet happy:

Then hie you hence to Friar Laurence' cell;
There stays a husband to make you a wife:
Now comes the wanton blood up in your cheeks,
They'll be in scarlet straight at any news.
Hie you to church (Act II, Scene 5).

Other than the nurse, Friar Laurence is the only other person who knows about Romeo and Juliet’s plans. He is concerned too, especially since Romeo was so recently head-over-heels for Rosaline. He marries them in secret. Once Romeo is banished, Friar Laurence gives Juliet a potion to fake her death so she will not have to marry Paris.

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:
And, if thou darest, I'll give thee remedy (Act IV, Scene 1).

I think you can definitely argue events would have turned out differently if Friar Laurence had not helped Juliet with the potion. She might have been unhappy, but she would not have died. Romeo and Juliet’s deaths are directly caused by that potion because Juliet wakes in her tomb to find Romeo has killed himself, thinking she really was dead. This prompts her to kill herself.