Regarding the insults Juliet throws at Romeo in Act III, Scene 2 of Romeo and Juliet, how can one write to either Romeo or Juliet, offering him/her advice about what he/she should have done?
First of all, the Nurse in Act III is characteristically unreliable and irrational in her reactions and words. For, she initially tells Juliet that Romeo is dead as well as Tybalt, generating Juliet's hysteria; then, she explains what she has meant by "dead." Above all, she gives Juliet no details on what has actually happened.
Therefore, in writing to Juliet, a person may wish to begin by explaining the cause of Romeo's rash action, refuting her words
Was ever book containing such vile matter
So fairly bound? O, that deceit should dwell
In such a gorgeous palace! (3.2.86-88)
The truth is that Romeo has been killed trying to ameliorate the argument between Mercutio and Tybalt; he has even told Tybalt that he loved him.
Another caution to give Juliet is not to react rashly as she has just done by the news from the Nurse, reminding her of her own previous words to Romeo in Act II as she urges Romeo to be patient and "swear not by the moon." She describes Romeo's actions, thusly,
It is too rash, too unadvised, too sudden,
Too like the lightning, which doth cease to be (2.2.124-125)
Thematically, the young people of Romeo and Juliet are impetuous and rash, while some adults urge patience and caution. Keeping this theme in mind while writing advice to Juliet (or Romeo) will lend verismilitude to the advice.