I was fortunate enough to be at the performance, as well. I was a few rows behind Ms. Lupone and across the aisle from Griffin Dunne, who was also at the performance. The audience treated both as private citizens, pretending not to acknowledge that they (specifically Patti Lupone) was in the audience.
It was only when Mr. Kushner, just prior to the curtain of Part Two, walked down the aisle to greet Ms. Lupone that we stopped pretending. It would be impossible not to acknowledge Mr. Kushner (especially after taking in Part One) at his own show. That is when the spontaneous standing ovation happened. Even for folks who did not recognize Mr. Kushner on site, when they saw Ms. Lupone - they joined in.
It was a lovely moment.
But the world goes 'round