What did Edith/Sydney whisper in Irving's ear when she barged into the meeting at the casino in American Hustle?
We need to talk! This is serious.
Carmine, Richie, the Sheik and Irving enter the back room of the Casino. Edith / Sydney stays at the bar to keep an eye on Rosalyn (who flirts with mafioso Pete). Then Edith / Sydney walks over to Rosalyn and they go to the bathroom.
Here's the scene and at around 2:19 min (right after De Niro), you see Edith / Sydney entering the back room and whispering into Irving's ear.
According to the script (130):
130 SYDNEY RUNS INTO BACK ROOM, URGENTLY JOINS IRV AND RICHIE AT THE TABLE. EDITH GREENSLY We need to talk! This is serious.
The scene ends afterwards and you see Carmine and Dolly climb on stage.