Jay-Z, 42, had invited them as a couple to the gig and is prepared to give Brown a second chance, providing he does not step out of line, reported Sun online.
"Jay-Z told Rihanna that if Chris is truly making her happy then to go for it.
"He invited them as a couple to his concert in New York and backstage he called Chris to one side and told him if he hurts Rihanna, whether emotionally or physically, it's over for him. He'll ensure it will result in the end of his career," a source said.
Brown, 23, who recently broke up with girlfriend Karrueche Tran, was convicted of assaulting Rihanna in 2009.
