Where does Friar Lawrence assume Romeo has been in Romeo and Juliet?

Friar Laurence at first believes Romeo is still pining over Rosaline, a woman who does not return Romeo's love because she says she has taken a vow of chastity (some think this means she has decided to become a nun). In fact, Romeo has completely forgotten about Rosaline and is now in love with Juliet, but since Juliet is the daughter of his father's enemy (and Rosaline was only his neice), this poses a major problem for Romeo.