In such a fanfiction scenario, Naruto would likely be extremely hurt. He has deep feelings for Hinata, so finding out she cheats would shatter his trust. He might initially go into denial, not wanting to believe it. Then, he could become angry and might distance himself from Hinata. His friends would probably try to console him, but it would take a long time for him to get over the betrayal.
He might be initially shocked. Naruto has been so focused on his goals like becoming Hokage that he may not have expected Hinata's love. He could be really touched that someone as kind and gentle as Hinata has had such deep feelings for him all this time.
She might become more confident. Since Naruto now knows how she feels, she doesn't have to hide her emotions as much. So she could be more open in her interactions with Naruto.
For Hinata, if she's actually innocent, she would be heartbroken that Naruto thought she could cheat. She might have to fight hard to prove her innocence. If she was at fault, she would likely face a lot of guilt and the loss of Naruto's love, which was something she deeply cherished. In the fanfiction world, this could also lead to changes in the social dynamics among their friends. Their friends might be divided, some believing Hinata and some believing Naruto's initial perception. It could create a rift in the whole group.
Another possibility is that the writer wanted to explore different character reactions. Naruto might initially be devastated. He could go through a phase of disbelief and then anger. However, if it's a well - written fanfic, there would likely be a resolution where the truth comes out and their relationship is either repaired or redefined.
She might be overjoyed. After all, Naruto is her son. She could immediately rush to find him and embrace him tightly, tears of happiness streaming down her face.
Well, one possibility is that Dark Naruto would be overjoyed. After all, Kushina is his mother. But at the same time, he could be very conflicted. His dark side might make him suspicious of her motives. He could test her in various ways to see if she's really who she says she is. And Kushina, on her part, would probably be heartbroken to see what her son has become.
Hinata would probably be extremely shy at first. She might blush furiously and be at a loss for words. After all, she has had a crush on Naruto for a long time, so it would be a dream come true for her.
Some of his other friends might react differently too. Some may be more cautious around him, worried about the power of the jinchuuriki within him. But those who truly trust Naruto, like Shikamaru, might not change their attitude much. They would see that Naruto is still the same person he was before, just with this added aspect to his identity.