Jessica Simpson and CaCee Cobb.
CREDIT: JOHN SHEARER / WIREIMAGE

It's only been six months since she split from Nick Lachey, but Jessica Simpson is facing yet another breakup: After two years, best friend CaCee Cobb recently resigned as the star's assistant to pursue behind-the-scenes work in the entertainment industry.

"Her job was always intended as a stepping-stone," a source tells PEOPLE in its upcoming issue, adding that the two are still in constant contact. "They support each other."

That hasn¡¯t stopped some publications from noting that Cobb¡¯s departure coincides with sightings of her partying with Lachey. The two chatted all night at Las Vegas's Pure on April 8 and ran into each other again May 1 at L.A.'s Shag.

Is Cobb falling for her best friend's ex? (Calling Denise Richards?) No! "They are friends and that's it," says a source close to Lachey.

"CaCee has always been friends with both Jessica and Nick," adds another source.

Lachey's brother Drew also strongly denies the romance rumors. "It's ridiculous."