The man could also be very supportive. He may join in the conversation later to reinforce the message his wife was giving. Say the wife was teaching the son about sharing, the man could later share his own experiences about sharing when he was a kid to further drive the point home.
In some stories, the husband may be conflicted. On one hand, he doesn't want to undermine his wife's authority, but on the other hand, he may not fully agree with spanking. So he might wait until after the incident and have a calm discussion with his wife about their parenting styles and how they can be more consistent and positive in disciplining their son.
One story could be that the wife calmly sits the son down and has a serious talk with him about his misbehavior. The man watches as she uses words to make the son understand the consequences of his actions. For example, if the son broke a neighbor's window while playing ball, the wife explains how it made the neighbor feel and that he needs to save up his allowance to contribute to fixing it.
She could be curious. Just seeing her husband stroking something might make her wonder what he's thinking or feeling at that moment.
The husband could also step in after the spanking to comfort the son while still reinforcing the lesson. He might say something like 'Your mother spanked you because what you did was wrong, but we still love you.' This helps the child understand the discipline was about the behavior, not lack of love.
One possible reaction is confusion. She may not understand why her husband is doing this in a situation where she can see. It could also lead to an argument if she feels it's inappropriate behavior in a shared space. Or, if she has a more liberal mindset, she might just casually ask him about it and start a conversation about personal boundaries.
One possible reaction from the husband could be silent approval. He might believe that the son has done something seriously wrong and that the spanking is a necessary form of discipline. For example, if the son has repeatedly disobeyed a safety rule, like running into the street without looking. Another reaction could be discomfort. He may not fully agree with the physical form of punishment but doesn't want to interfere immediately in front of the son, so he just stands there uneasily. Or he could be empathetic towards his son. He might try to comfort the son later in private and also have a conversation with his wife about alternative forms of discipline.
They might be amused. Since fanfiction often takes the original characters and puts them in new and sometimes humorous situations. For example, the serious Allen Walker might find himself in a comical scenario in the fanfiction, which could make him crack a small smile if he were real and watching it.
He could have different reactions depending on their family's discipline philosophy. If they generally agree on strict discipline, he may support his wife's action silently. However, if he believes in more gentle ways of teaching, he might step in later to comfort the son and have a talk with his wife about alternative methods.
They might be surprised at how creative the fans are. Some characters could be amused by the different interpretations of their personalities and the events in Endgame. For example, if Iron Man's character reads a fanfiction where he has a different ending in Endgame, he might find it interesting to see how fans envisioned an alternative scenario for him.
Others could be really intrigued. They might get caught up in the new storylines and start to wonder what if these things could actually happen in their universe. Like Arnold, he's always open - minded, so he might be really into exploring these new fan - created scenarios.