04/11/2008 at 05:00 AM EDT
Like many of Jay-Z and Beyoncé Knowles
's friends and colleagues, Rihanna
had no idea about the happy couple's hush-hush nuptials
"I didn't know!" Rihanna told PEOPLE at the Escada Moon Sparkle/Island Def Jam benefit for her Believe Foundation and DKMS, the world's largest bone marrow donor center. "No one knew!"
(Although the wedding has not been officially confirmed, sources tell PEOPLE the ceremony did take place April 4 and Knowles has been spotted wearing a huge ring
Among those in the audience at Manhattan's Highland Ballroom were L.A. Reid and Rihanna's "friend" Chris Brown. "Chris said it was a good show," the 20-year-old said of her rumored beau
. "He really enjoyed it."
After the show, Rihanna met with several young bone marrow transplant patients. "You are so beautiful," she told 15-year-old Camila Mera.
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