Demi Lovato's Upcoming Role: Bridesmaid

04/27/2011 at 01:35 PM EDT

Demi Lovato's Upcoming Role: Bridesmaid
Tiffany Thornton and Demi Lovato
Wireimage; INF

Demi Lovato

Demi Lovato

Tiffany Thornton has been there for Demi Lovato lately. Soon, Lovato will get to repay the favor.

"Demi is one of my bridesmaids!" Thornton, 25, who got engaged this winter, tells PEOPLE of her costar on Disney Channel's Sonny with a Chance. "We go way back. She's like a little sister to me. She's one of my closest friends."

Thornton has been supporting Lovato, 18, during her treatment for eating disorders and bipolar disorder in recent months. That means texts, tweets and good old-fashioned girl time.

"We love Pinkberry!" Thornton said at Tuesday's Voices of Meningitis luncheon in New York. "We hang out a lot, we talk about our faith. I just really enjoy spending time with her."

The two have been pals since they started filming Sonny with a Chance in 2009. Thornton's sisterly concern for Lovato is clear. "You could tell she was always tired," she said. "She works more than any person I have ever known in my entire life."

Lovato recently said she won't return to Sonny with a Chance, so she can focus on her music career. In that, she has the full support of her Disney Channel friends.

"I'll miss Demi," says Thornton. "I'm really proud of her. It takes a lot of guts to admit that you aren't perfect in front of the world."

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