Aspiring singer Sims was served on leaving a party at a private home in the Hollywood Hills. A shocked looking Sims was seen shortly after in tears as she sat in her car.
A source told People: "Alli was really shocked and covered her face".
"She was really distraught and said 'Oh my God.' "
Federline's lawyer Mark Vincent Kaplan confirmed that Sims, Britney's longtime party pal, would give a statement "in connection with the Britney Spears – Kevin Federline issue".
He said the subpoena was delivered to Sims because she was "believed to have relevant information". Britney was reported to have been at the same party as Sims, but did not witness the serving of the papers.
She left the party hand-in-hand with a mystery man looking worse for wear, and at one point, tearful.
