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.
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.
Natsu might be initially shocked and confused. He's not the type to expect rejection, so he could be at a loss for words at first. For instance, he might just stand there with his mouth open, not believing what he just heard.
Natsu would likely be extremely hurt. He has always been loyal to Fairy Tail, so being betrayed would be a huge blow. He might initially be in disbelief, not wanting to accept that his friends and guildmates could turn on him.
He could also react with confusion. Natsu is a bit of a simple - minded character in some ways, and he might not understand why someone would want to bully him. This confusion could lead to him trying to figure out the bully's motives.
She might become angry at first. Natsu is known for her fiery temper. So, she could lash out at the person who betrayed her, confronting them directly and demanding an explanation.
Natsu's friends would have a range of reactions. Lucy would be distraught, her face pale with fear as she kneels beside Natsu. Gray would clench his fists, his eyes filled with rage as he scans the area for the shooter. Erza would be the voice of calm and reason, though her hands would be shaking slightly as she tries to stop the bleeding. Happy would be flying around in a frenzy, screeching in distress. They would all be united in their determination to save Natsu and seek justice for what has happened.
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.
Their friends would likely be very excited. Lucy would probably be overjoyed and offer to help with things like baby clothes shopping. Gray might act a bit gruff on the outside but secretly be happy for them. Erza would be very protective, making sure that Lisanna has everything she needs and that the guild is a safe environment for the baby. Happy, Natsu's Exceed friend, would be constantly around, making cute comments about the baby and offering his own unique brand of support.
He might initially be shocked and confused. Natsu has always been so loyal to Fairy Tail, so this would be a huge blow to him.