Capulet asks Nurse where Juliet has gone in the beginning of Act IV, scene 2. The Nurse indicates that she went to Friar Lawrence for confession purposes. Capulet responds by saying, "Well, he may chance to do some good in her. A peevish self-willed harlotry it is."

Even if Capulet was unaware of Juliet's marriage to Romeo, what does this quote suggest he was aware of?

It suggests he's aware of Juliet's "bad" behavior. He's basically saying that the Friar could still possibly "change her bad behavior". You know, something like thaat.

The quote suggests that Capulet is aware or suspects that Juliet has engaged in some kind of inappropriate behavior, which he describes as "peevish self-willed harlotry." While he may not be aware of the specifics of Juliet's marriage to Romeo, he clearly believes that she has done something that he disapproves of.