He might be angry at first. Natsu is a passionate character, so he would likely feel betrayed and lash out immediately.
Lucy might be heartbroken. After all, Team Natsu was like a family to her. She could initially go into a state of shock and disbelief. Maybe she would try to plead with the team to let her back in, explaining that whatever they thought she did was a misunderstanding.
Maybe he violated some important team rules. For example, he might have gone against the orders of the team leader during a crucial mission.
One possible reason could be a misunderstanding. Maybe there was a miscommunication during a mission, and the other members of Team Natsu thought Lucy made a big mistake. For example, if she accidentally led the enemy to their hiding place. Another reason might be jealousy. If Lucy's celestial spirit magic started to gain too much attention and overshadowed the others in some way, they could have decided to kick her out. However, this is all just speculation based on the idea of this fanfiction scenario.
Natsu would probably be very angry at first. He loves Fairy Tail like a family, so being kicked out would feel like a huge betrayal. He might go on a rampage, shouting and arguing with the guild members who made the decision.
He might be really angry at first. Natsu is a passionate character, and being kicked out unjustly would make him furious. He could storm off vowing to prove his worth and show the guild they made a mistake.
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 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 guilty. He might immediately stop whatever he was doing and rush to Lucy's side.
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.
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.