Friar Lawrence is thinking about he has to quickly get word to Romeo. He intends to send another friar with a message to Romeo. He knows the potion he has given to Juliet will be taken Wednesday night and that once injested it will take 42 hours for it to completely run its course.
Before the wedding between Paris and Juliet was part of any one's plans, this wouldn't have been an issue, but now, the Friar has to orchestrate the blazoning of Romeo and Juliet's marriages craftily or get Juliet the heck out of Verona.
Juliet is too immature to carefully plan through this situation. She hasn't had the experience of adult trauma. As in most life circumstances, she has had an adult to rely on. Here, because her parents have no idea about her relationships with Romeo, she relies on Friar Lawrence.