Nicole Richie has accused longtime boyfriend Joel Madden of doing the mattress mambo with her best friend-none other than out-of-work actress Mischa Barton.

Nicole is ready to beat a hoe down over reports that Joel and Mischa partied up a storm at Hollywood’s posh Chateau Marmont while she was enjoying runway shows during New York Fashion Week September 7.

Tabloid spies say Mischa danced wildly in front of Joel as he jammed with her rock band Good Charlotte.

“She was wearing a very revealing white dress that left little to the imagination,” a Star insider says. “Mischa is into the bad-boy rocker type, and it was pretty obvious by the way she was flirting the she was getting turned on by Joel.”

“Joel kept checking her out and they traded smiles every time they made eye contact. Her flirting was obvious to everyone.”

Luckily for Nicole, Paris Hilton, who was in the crowd cheering on boyfriend Benji Madden, wasted no time letting her on-again/off-again pal know what was up.

“Paris was texting Nicole updates on Mischa and Joel, and even sent photos from her cell phone of Mischa dirty dancing.”

The source adds: “Mischa was very touchy-feely, petting Joel’s arm and leg, and giggling as they sat and talked.”

The two later left the club together.

Nicole tearfully confronted Joel upon her return from the Big Apple.

“Nicole feels he betrayed her trust. She told Joel that she considered his behavior cheating.”