K-FED GETS FIERCE!

At 2:30 a.m. on Sunday, Britney Spears's longtime pal, Alli Sims, was served with legal papers concerning the ongoing custody battle between Spears and her ex-husband, Kevin Federline, a source tells PEOPLE.

Sims was leaving a party at a private home in the Hollywood Hills when she was served by Aaron Cohen of IMS-Security. "Alli was really shocked and covered her face," an observer tells PEOPLE. "[She was] really distraught and said 'Oh my God.' " Spears, who was also at the party, did not witness the serving of the papers, according to the observer.

The papers are a deposition subpoena sent by Federline's lawyer, Mark Vincent Kaplan, compelling Sims to testify about Spears's parenting skills. Sims, who has known Spears since childhood and is regularly at her side while partying in Los Angeles, recently told PEOPLE: "[Britney] is a wonderful mom and she is so hands-on. ... Britney loves those babies to death. And they adore her."

On Wednesday, Federline filed for primary custody of the couple's two sons, Sean Preston, 22 months, and Jayden James, 10 months. A source close to Federline told PEOPLE recently that the would-be rapper has been worried about Spears's behavior and thinks she may be exposing the boys to "unnecessary risk."