Lydia's wild behavior has indeed scared off some very eligible men for his two eldest daughters. And I find it hard to believe that even Mr. Bennet would be unaware that if Lydia misbehaves in Brighton, it could have a damaging effect on the reputation of the whole family. He tries to joke Lizzy out of being upset, but he doesn't ask her for specifics about Lydia's behavior. It doesn't occur to him that a sister might observe things a parent wouldn't. Maybe Lydia is even better behaved around her father. Certainly he isn't with her when she goes parading around Meryton in search of officers.