Yes, it can be. In some fictional works, such as certain mystery or drama novels, this kind of plot can be used to create tension and conflict. It plays on the idea of betrayal and the close proximity of the unfaithful act to the unsuspecting spouse, which can be a very powerful narrative device.
Morally, it's clearly wrong. Cheating in general is a breach of trust in a relationship, and doing it right next to her husband is even more callous. It shows a lack of respect for her partner and the commitment they have made to each other.
Some signs could be being overly secretive with her phone, suddenly having new and unexplained interests or hobbies that take her away from home often, and being less affectionate towards her husband.
The husband might choose to give his wife some space at first to see if she'll come clean on her own. But he should also gather his thoughts and evidence if he suspects something. He could also seek advice from a close friend or family member he trusts.
A wife cheating on her husband in a revenge novel usually involved the following plot:
The protagonist was cheated on by his wife. He felt angry, disappointed and desperate, and decided to take revenge.
Through various means, including investigation, stalking, threats, etc., the protagonist finally succeeded in exposing the truth of his wife's adultery, making her feel guilty and ashamed.
After revenge, the protagonist would usually seek inner balance and repair, try to communicate with his wife and try to forgive her mistakes.
In some novels, the protagonist may choose to divorce or take other actions to end the marriage, but after revenge, he will usually reconsider his life choices and start a new life.
This kind of novel usually involved topics such as love, marriage, family, morality, etc., which might bring readers deep thoughts and enlightenment.
Some signs could be sudden changes in her appearance like dressing more provocatively. Also, if she starts being overly secretive with her phone, not allowing her husband to see it. Another sign might be she often comes home late without a good explanation.
He can keep a journal of the things that are making him suspect her. Write down the dates, times, and what exactly happened. This way, he can have a clear record if he ever needs to address the issue further. At the same time, he should try to maintain a normal life as much as possible. Don't let the suspicion completely take over. Try to still have normal interactions with his wife and family, but be observant at the same time.
Another sign is being overly protective of her phone or computer. In many stories, the cheating wife doesn't let her husband near her devices. She might quickly close apps or turn off the screen when he comes close. This is because she doesn't want him to see the messages or photos from the other man.
One possible reason could be a lack of satisfaction in his own marriage. Maybe he feels unappreciated or neglected by his wife. Another reason might be the proximity and familiarity with the wife's friend. They might have spent a lot of time together, which could lead to the development of inappropriate feelings. Also, some people have a tendency to seek novelty and excitement outside of their committed relationships.
First, he should try to stay calm. Reacting impulsively might make things worse. He could start by having an open and honest conversation with his wife about what he suspects.