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.
Tiffany 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."
Tiffany Thornton announced her engagement last February (10).