The two "Sonny with a Chance" stars have become really close after Thornton helped Lovato as she struggled through a very public meltdown in recent months, and now Lovato will get a chance to show her pal her appreciation and step in to support her on her big day.
She tells People.com, "Demi is one of my bridesmaids! We go way back. She's like a little sister to me. She's one of my closest friends."
But the two actresses are co-stars no more after Lovato recently quit the show to focus on her own physical and emotional recovery.
Thornton adds, "I'll miss Demi. I'm really proud of her. It takes a lot of guts to admit that you aren't perfect in front of the world."