The actress and her boyfriend Sean Friday were spotted supporting the 25-year-old as she took the stage at The Sayers Club nightclub in Hollywood to sing a cover of songs on Tuesday night.
"Demi looked so proud watching Rumer's set. Rumer wore a deep-V, long black dress and sang about five songs," a source tells E! News. "The set was very soulful, and the crowd was really into her performance."
Rumer even took on a Miley Cyrus hit!
"The crowd went wild when she covered 'Wrecking Ball' as the last song. Rumer and the band were really talented, and it was obvious Demi was really proud of Rumer. She was watching Rumer intently, and smiling as she performed. Demi would clap after each song, and was focused on watching Rumer perform," the source adds.