Australian pop star SOPHIE MONK is dating Good Charlotte singer Benji Madden - and she has won the approval of teen queen Hilary Duff, the long-time girlfriend of his twin brother BENJI. While they've yet to become sisters-in-law, the blonde twosome have struck up a firm friendship since Monk began dating Benji. Duff tells the Sydney Daily Telegraph, "She's a really nice girl. She's really sweet and she's beautiful, but I can't say anything more because I don't know how they want to handle that." The WHY NOT singer admits she and Joel initially kept their involvement a secret for more than six months - because he was 24 at the time and she was only 16. She adds, "We didn't want anyone to know we were dating because he's much older than I am and it's a perfect opportunity for people to talk about you negatively."