The "Rehab" singer – who was dressed in a black Dolce and Gabbana power suit – joined Kanye West on stage at the Metropolitan Museum of Art Costume Institute Gala in New York on Monday (May 4, 2009).
The pair performed a duet of Kanye's song "Say You Will" before Rihanna belted out an emotional rendition of her hit "Live Your Life."
A source said: "Kanye asked her to do it. It was his idea, but Vogue editor Anna Wintour obviously had to approve it. Anna was at the run-through during the day and was so into it."
The audience applauded wildly when the 21-year-old star walked on stage, with many shouting messages of support.
Rihanna did not respond to questions about Chris, but smiled and waved at the crowd.
After her performance, the star was seen chatting to Madonna, Victoria Beckham, and fashion designers Marc Jacobs and Stella McCartney.
She was later seen talking to "Gossip Girl" star Blake Lively at Jacob's after-party at trendy restaurant Monkey Bar and spotted dancing with friends at nightclub Bungalow 8.
Chris, 20, pleaded not guilty to assault and making criminal threats in a Los Angeles court last month.
