Well, in many fanfictions, Natsu might initially be in shock. He's a passionate character, so he could react with anger. He might lash out at Lisanna or the person she cheated with. He could also go into a state of self - doubt, wondering what he did wrong.
Their relationship often develops through shared experiences. For example, they might go on a quest together. During the quest, they face challenges and have to rely on each other. This builds trust between them. Natsu might start to see Lisanna in a new light, not just as an old friend but as someone he could have deeper feelings for. Lisanna, on the other hand, might become more confident around Natsu and show her true self more often.
In some fanfictions, Natsu might save Lisanna from a powerful dark wizard. He could use his Fire Dragon Slayer magic to overpower the enemy and rescue her.
As they travel, their bond deepens. They start to rely on each other more. Natsu becomes more protective of Lucy, and Lucy begins to understand Natsu's impulsive nature better.
Lisanna might face a lot of backlash from the guild. Her friends could be disappointed in her and might not trust her as much. She could lose her standing in the guild and have to work hard to regain people's respect.
Lucy could face isolation. People in the fanfiction world might avoid her, and she could be left all alone. This could also lead to her losing confidence in herself as she is constantly being blamed for something she didn't do.
Writing fanfic with sexual content is generally not appropriate, especially when it involves characters from a mainstream series. It goes against the family - friendly nature of many of these works and can also be disrespectful to the original creators and the fan community at large.