Shannon Funk was served late Tuesday night at the airport in Long Beach, Calif.
The first to be served was Spears’s childhood friend, Alli Sims, 26, who received her subpoena at 2:30 a.m. on Sunday as she was leaving a private party in the Hollywood Hills, which was also attended by Spears.
Next on the list was the singer’s bodyguard and “manny,” Daimon Shippen, who was served late Monday night as he was getting out of his car in Santa Monica, Calif.
Federline’s lawyer, Mark Vincent Kaplan, is behind the subpoenas, which will be used to compel those close to Spears to discuss her parenting style.
Last week Federline filed for primary physical custody of the couple’s two sons, Sean Preston, 23 months, and Jayden James, 11 months.
Currently, Federline, 29, and Spears, 25, share joint legal and physical custody of the children.