Jessica Simpson's other breakup

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."