What is the test for appointing a guardian ad litem in a federal court?

The appointment of a guardian ad litem pursuant to Rule 17(c) is committed to the sound discretion of the district court. See United States v. 30.64 Acres of Land, 795 F.2d 796, 805 (9th Cir.1986). "A guardian ad litem is authorized to act on behalf of his ward and may make all appropriate decisions in the course of specific litigation. For example, notwithstanding the incompetency of a party, the guardian may make binding contracts for the retention of counsel and expert witnesses and may settle the claim on behalf of his ward." Id. Moreover, "federal, not state law, governs the appointment of a guardian ad litem in federal court." Hulstedt v. City of Scottsdale, Arizona, No. CV-09-1258-PHX-MHM, 2011 WL 772387 at *2 (D. Ariz. Mar. 1, 2011). See also Gibbs v. Carnival Cruise Lines, 314 F.3d 125, 134-35 (3rd Cir. 2002). Thus the court's determination "focus[es], as it must, on the best interests of [the plaintiff]." Id.