Federline attorney Mark Vincent Kaplan told The Associated Press that Alli Sims would be asked to give a deposition "in connection with the Britney Spears-Kevin Federline issue," but would give no specifics.
He said the subpoena was delivered to Sims because she was "believed to have relevant information," but declined to discuss what she would be asked.
It was unknown if the testimony was connected to the child custody hearing scheduled for Aug. 14.
Spears spokeswoman Gina Orr did not immediately respond to an e-mail seeking comment Sunday.
Spears' divorce from rapper and dancer Federline became official July 30.
Since February, Spears, 25, and Federline, 29, have shared joint custody of their sons, 22-month-old Sean Preston and 10-month-old Jayden James.