What page number this incident is on depends upon the edition of Night you are using. In the 2006 edition translated by Marion Wiesel, this scene occurs on page 54. If you are using another edition, know that the scene occurs after the episode with the Aryan-passing Jewish woman Elie re-encounters in the Paris Metro years after the war.
In the incident mentioned, Elie watches passively as his father, Chlomo, is beaten by Idek the Kapo. Elie mentions that Idek was already on edge and the target for his frustrations that day just happened to be Elie's father. Brandishing an iron bar, Idek beats Chlomo without mercy.
Elie does not intervene and undergoes a flurry of emotions, not all of them anger at Idek. In fact, Elie feels a great deal of negative emotion towards his father. He feels Chlomo is weak and wishes he had somehow been clever enough to avoid being Idek's chosen target altogether. Elie is even tempted to run away from the sight of his father being abused, only so he will not be the next target.
Elie not intervening in Chlomo's beating is an early incident of how life in the camps has hardened him. His ability to feel compassion for others is blunted by despair, anger, and self-preservation. Elie comes to feel great guilt over this and tries to help his father more in the future, even though he still struggles with his anger and exhaustion.
The incident you are referring to takes place on page 54. This particular beating took place during a manual labor session in which Elie and his father were among a group of men “loading diesel motors onto freight cars”.
Idek is a kapo, which is a prisoner who is assigned to supervise labor. He runs Eliezer’s work unit and is described as being “on edge” on the day of this particular beating. It is well-known that on the days when he is in a crazy mood, Idek is a force of evil to be reckoned with. Suddenly, Idek explodes, and it is Elie’s father who faces the might of his wrath.
After accusing Elie’s father of being an “old loafer,” Idek starts beating him with an iron bar. Elie watches from a distance as his father is beaten. By this time, he is so worn down by the system that he actually feels anger towards his father at that moment. He finds himself wishing that his father had behaved differently so that Idek would not have chosen him to pick on.
Elie acknowledges that in his former life, he would not have reacted this way. He tells readers that “this was what life in a concentration camp had made of [him].” Everything about life at Birkenau was unspeakably cruel. Life here has robbed Elie of his natural instinct to protect his father.
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.