I can see where some confusion could arise, The Soviet Union and France sections discuss a gain of additional pieces that are separate from NOs. So a discussion of IPCs under the United States section could logically imply a separate “gain” of IPCs. Especially since it names this additional IPC as “wartime economy” and states that the U.S. gets this wartime economy on the first turn after an axis D.O.War or the third turn of the United States whichever comes first…indicating that it is a one time thing gained on turn 3 or sooner if attacked.
Since you have your official answer, you are now aware that they receive +30 to the Production chart while at war, thus turn 3 you would collect 35+30 if you did not gain any other territories on round 3.
It would be less confusing if it were not mentioned twice in the rule book I guess. Your friend is schrewd for making that argument. As it might have worked on me, despite my familiarity with the game.