Miley Cyrus on American Idol + New MileyWorld Video

Demi Lovato’s Upcoming Role: Bridesmaid

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.”

Miley & Tish Cyrus @ LAX

Miley Cyrus and her mom Tish were spotted at Los Angles International Airport today. (April 27) They are currently on their way to Ecuador, where Miley will begin her Gypsy Heart Tour on Friday, April 29!

In other news, Miley will make a surprise appearance on American Idol tonight:

An American Idol finalist comes face-to-face tonight with one of her, well, idols.

Lauren Alaina, who belted out Miley Cyrus’ “The Climb” a few weeks back, gets some guidance during Wednesday’s performance show from none other than the teen sensation herself.

Idol sources tell E! News exclusively that Cyrus will make a surprise appearance as a mentor for Alaina alongside Interscope’s Jimmy Iovine, who has been assisting the 16-year-old contestant and the other young hopefuls throughout the competition.

Recently, Iovine caused quite a stir among Cyrus fans when he told Alaina while rehearsing “The Climb” that she was “a much stronger singer than Miley Cyrus.” An Idol source says Iovine promptly invited Cyrus, of whom he has been a longtime fan, to be a mentor because he regretted making the remark.

“Jimmy apologized and personally asked Miley to come one,” says the show source.

The song Alaina is scheduled to sing this week will be, according to a source, “a mid-tempo ballad.”

Cyrus, who has appeared on Idol in the past, taped the mentoring session some time late last week.

More pics HERE.