He marries Romeo and Juliet in secret.
He provides Juliet with the potion.
These two actions of Friar Laurence contributed to the catastrophe in Romeo and Juliet. If Romeo and Juliet were never married, Juliet would not have felt trapped and desperate if she hadn’t been married to Romeo. Also, if Juliet hadn’t appeared to be dead, Romeo would not have taken his own life.
The statements “He convinces Juliet not to marry Paris.” and “He reveals to Romeo that Juliet is dead.” are incorrect. He never did these things. Friar Laurence did inform the families about the marriage but it wasn’t until after the catastrophe.