Because he knows that Juliet is married to Romeo. He was the one who married them.
Juliet asked Friar Laurence to help her avoid marrying Paris by giving her a plan to fake her death so she could be with Romeo instead.
Romeo proposes to Friar Laurence that he and Juliet be married in secret, without the knowledge of their feuding families. Romeo seeks the Friar's help in marrying them to bring peace between the Capulets and Montagues.
Friar Laurence hopes that his plan for Juliet to fake her own death will allow her to avoid marrying Paris and ultimately reunite with Romeo. He believes this will bring peace and happiness to both Juliet and Romeo, ending the feud between their families.
Romeo goes to friar Lawrence about marrying Juliet
Romeo asks friar Laurence to marry him and Juliet in secret
Juliet asks the Nurse to listen to her confession instead of Friar Laurence.
Friar Laurence married off Romeo and Juliet.
Friar Laurence is the friar who marries Romeo and Juliet.
Friar Lawrence married Romeo and Juliet in his cell.
friar Laurence
The Nurse and Friar Lawrence knew about it before anyone else because the Friar performed the marriage and the Nurse was Juliet's close confidant so she told her everything.
to help marry him and Juliet