On what page in the book Night by Elie Wiesel did Elie's father get beaten while Elie just stood there doing absolutely nothing, with not a...

Idek beat Wiesel’s father for “loafing” while working, and he did nothing to stop it.

In the instance I believe you are describing, Elie Wiesel watched the Kapo Idek beat his father for no reason, because he said he was “loafing” (not working fast enough) while loading a boxcar. In this case, they were “loading diesel motors” onto the freight cars, and in addition to Idek they were also being watched by German soldiers. Perhaps this is why Idek was particularly grumpy. 

I had watched it all happening without moving. I kept silent. In fact, I thought of stealing away in order not to suffer the blows. What's more, if I felt anger at that moment, it was not directed at the Kapo but at my father. Why couldn't he have avoided Idek's wrath? (Ch. 4, p. 54) 

At the time, he did nothing because he was angry at his father for drawing Idek’s attention, but he was also angry at the Nazis and the concentration camp environment for turning him into a person who could think this way. He knew it was wrong. He still felt empathy for his father, but he was helpless to stop the beating and he knew it. That was the real source of his anger. 

It was not the only time he could not protect his father. Elie Wiesel later regretted watching the SS beat his father while he stood by and did nothing. In the introduction, he says, “I shall never forgive myself.” However, he also acknowledges that it was the concentration camp that pushed him to this point. 

Nor shall I ever forgive the world for having pushed me against the wall, for having turned me into a stranger, for having awakened in me the basest, most primitive instincts. (Introduction, xii) 

Wiesel’s indifference in these moments opened his eyes to the larger problem of the Holocaust. Too many people were ignoring what was happening, just as he ignored what happened to his father. His father knew that there was nothing he could do and did not blame his son, but he would have still been hurt that his son was angry at him for being a target. The concentration camp was a hellish environment that made people do terrible things to each other.