Don’t invite Nicole Richie and Hilary Duff to the same party. Duff has bad-mouthed Richie who she thinks is a “skank,” according to Us Weekly.

Joel Madden is apparently at the center of the friction. Madden and Duff split in November, and although Madden has been loyal to Richie, he keeps calling his ex, according to the mag, but she doesn’t return his call.

“[Duff] can’t believe he’s dating Nicole and thinks she’s a skank,” a source told Us, adding that when Duff heard about Richie’s arrest, “she asked if Nicole had been coming from Joel’s. ... She kept saying she never knew Joel at all.”

Labels: Hilary Duff, Nicole Richie, The War is On