Some of his more loyal Pokemon might try to resist the change. For example, Pikachu could use electric shocks to try and jolt Ash back to his senses. Pikachu has a strong bond with Ash and would be very confused and sad about his decision to join Team Rocket.
Pikachu would probably be very confused at first. It's used to fighting against Team Rocket, not being on their side. It might resist and try to zap Ash to snap him out of it.
Pikachu would likely be very confused and resistant. Pikachu has a strong bond with Ash and knows his true nature. It would probably try to shock Ash out of whatever trance or false belief he has from joining Team Galactic. For example, when Ash starts to follow Team Galactic's orders that go against his usual principles, Pikachu might refuse to cooperate and try to show Ash that he's on the wrong path.
In a fanfiction where Ash joins Team Rocket, it could lead to some really interesting storylines. Ash has a lot of knowledge about Pokémon training that Team Rocket lacks. He could potentially teach them new techniques, but at the same time, his moral compass would be constantly at odds with their devious ways. Maybe he joins them because he's somehow tricked or he has his own secret plan to take them down from the inside. It would be a great opportunity to explore different aspects of his character and how he adapts to this new situation.
Well, if Ash joins Team Rocket in a fanfic, it would be a huge shift in the Pokemon world. His friends, like Misty and Brock, would be shocked. They might try to rescue him. Ash, on the other hand, could initially be attracted to the idea of a new adventure with Team Rocket. He could see them as underdogs in a way, always failing but still trying. But as time goes on, he'd start to see the problems with their way of doing things.
One idea could be that Ash feels betrayed by his friends because they always overshadow his achievements. So, in a fit of anger and disappointment, he decides to join Team Rocket. He might think that with their resources, he can finally become a more powerful trainer on his own terms.
Ash might be heartbroken at first. He'd feel a deep sense of loneliness and confusion.
Well, it could make for a really interesting fanfiction. Dawn joining Team Rocket would change the whole dynamic. Her Pokémon might not be too happy about it at first. She could bring new ideas to Team Rocket, since she has a different way of thinking compared to Jessie, James and Meowth. However, it's also possible that she would secretly be working against Team Rocket from the inside, trying to take them down while pretending to be one of them.
Ash joining Team Rocket could lead to some interesting character dynamics. He might start to question his own values. For instance, he's always been about friendship with Pokemon, but Team Rocket is more about using them for gain. This internal conflict could be a major part of the fanfic. Also, his relationship with Jessie, James, and Meowth would change drastically.
The Pokémon might initially think it's some sort of joke or mistake. But as time goes on, if Ash seems serious about being with Team Rocket, they could become divided. Some of the more headstrong Pokémon might leave Ash, while others might stay with him out of loyalty but still be unhappy about the situation. They would be constantly trying to nudge Ash back to his old ways of being a good Pokémon trainer.
I think Ash would be angry as well as hurt. He's a passionate trainer and his friends mean a great deal to him. He might challenge May in a Pokémon battle, not to defeat her but to make her see how wrong her betrayal is. And during the battle, he would keep asking her to come back to their old friendship.