Thank you for that. So there you go: As I read it, USADA is a state actor in this case under 3, 4, and 5 above.
"3. If the government merely acquiesces in the performance of an act by a private individual or organization it is not state action, but if the government coerces, influences, or encourages the performance of the act, it is state action (Rendall-Baker v. Kohn, 457 U.S. 830 (1982)"
The mere presence of government investigators in an interview, nevermind a trip abroad, would tend to encourage and passively influence USADA and its investigator(s).
"4. If the government merely enters into a contract with an individual or organization for the goods or services, the actions of the private party are not state action, but if the government and the private party enter into a “joint enterprise” or a “symbiotic relationship” with each other it is state action (Burton v. Wilmington Parking Authority, 365 U.S. 715 (1961)"
It's hard to see how joint interviews and trips abroad would not constitute a "joint enterprise", even where one party was there merely as an observer, or how it would not create a "symbiotic relationship", where the parties had interests in common.
"5. If government agencies are simply members of a private organization, the actions of the organization are not state action, but if the government is "pervasively entwined" with the leadership of the private organization, the acts of the organization are state action (Brentwood Academy v. Tennessee Secondary School Athletic Association, 535 U.S. 971 (2002)."
If Travis Tygart has been consulting with, or even merely observing on a regular basis, these various interviews, it should be pretty easy to make the case for "pervasively intertwined".