The DNCE frontman briefly dated his Camp Rock co-star in early 2010, just months before she lost her temper and punched a dancer during her joint tour with the Jonas Brothers.

Demi subsequently entered rehab to seek treatment for her battle with drugs and alcohol, while she also suffered from depression, an eating disorder and self-harm. But the pop star has maintained her sobriety and been at the top of her game with a solid career in music, something of which her former beau Joe is extremely proud of.

"She's the best version of herself I've ever known," Joe tells Billboard magazine. "She's healthy and that's obviously something a lot of the world knows now, but she's gotten over the really hard part. I know a lot of friends who've gone through similar things and it's when you have to live with your new self for a few years - that comes with a lot of struggle.

"And being a pop star on top of that, that's a lot of pressure, but she's amazing at speaking truth to what she believes. I wish I could have a bit more of that. Excuse my language, but she doesn't give a f**k. She goes out there and says, 'This is who I am.' Considering what we've all come from, it's not always easy. We feel like we have to be cautious."

Demi, who recently split from long-time boyfriend Wilmer Valderrama, has also stayed close with Joe's younger brother Nick - a dynamic Joe suggests is even stronger than the one he has with his sibling.

"You'd think they're siblings, sometimes more than Nick and I," he reveals. "They pick on each other, they play games... Demi's a great listener, and she keeps Nick smiling - she'll always make a serious situation laughable. So, they've found great support with one another and they lean on each other creatively."

Since Demi has become a member of their extended family, Joe admits she is just as nosy about his private life as a real sister would be, explaining, "Demi will overhear me and Nick talking and she'll be like, 'What are you talking about?!' So she likes the gossip. She likes hearing about my girl drama. I'm always like, 'I wasn't talking to you, Demi.'"