The 'Run It' singer - who was convicted of assaulting his then-girlfriend in 2009 - reportedly delighted his former flame by turning up to the bash she hosted at her Beverly Hills home on Monday (13.02.12).
Guest Katia C tweeted: "If Cupid was here he would be very happy everyone @Rihanna bday party was coupled up.
"All I can say, CB was invited and she was acting like the happiest girl on her birthday and probably got what she asked for."
Rihanna - who turns 24 on February 20 - spoiled guests including Kelly Rowland and Bruno Mars with champagne, mermaid vodka ice sculptures, a Caribbean banquet, Cuban cigars, which cost £35 each, and 200 cupcakes and she and newly-single best friend Katy Perry particularly enjoyed the festivities.
A source told the Daily Mirror newspaper: "Rihanna and Katy had everyone going crazy. By the end of the night they started a cake fight and were dancing together while Bruno and Chris looked on open-mouthed, cheering.
"Ri made sure Chris was having a good time but then he kept a low profile, spending most of the night in a corner with a few pals."
However, Katy and Rihanna's outfits fell victim to their partying.
The source added: "At one point Katy danced so much she broke her heels and had to leave the party barefoot.
"Rihanna also had her own wardrobe casualty when her corset came undone but she didn't let that stop the fun."