His Pikachu would probably be very supportive. It would stay by his side as he studies, maybe even help him in small ways like fetching medical tools. It trusts Ash completely and would be happy to see him pursue something new.
Some of his more battle - hungry Pokemon like Charizard might be a bit confused at first. But once they see how Ash's new role helps other Pokemon, they would come around. They might even protect the Pokemon center where Ash is training.
Ash's Pokemon would likely have mixed reactions. Some like Bulbasaur, being the gentle and observant one, would be quick to accept Sabrina. It might even enjoy the new psychic powers that Sabrina could bring into their group. However, Squirtle might be a bit mischievous at first, trying to test Sabrina's patience. For Sabrina's Pokemon, Alakazam, being highly intelligent, would sense Ash's good intentions towards Sabrina and would support the relationship. Her other Pokemon would follow Alakazam's lead.
Pikachu might be very confused at first. When it sees 'Ash' (really May in Ash's body) acting differently, it could be hesitant to follow commands as usual. For May's Pokemon, they might be surprised to see 'May' (Ash in May's body) suddenly having a more battle - oriented attitude. For example, May's Beautifly might be confused when 'May' wants to train it for battles instead of focusing on its appearance for Contests.
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.
He would likely be heartbroken. Ash is a very emotional character. He would be confused and might start to question everything he knew about his mother. He could also try to resist in a non - violent way, hoping to bring his mother back to her normal self. He might call out to his Pokemon for help, not to attack his mother but to try and restrain her until he can figure out what's going on.
Ash might be heartbroken at first. He'd feel a deep sense of loneliness and confusion.
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.
Ash would be extremely hurt. He's a very loyal person, and having someone he cares about like Lillie betray him would be a huge blow. He might initially go into denial, not believing that Lillie could do such a thing.
Pikachu would be super excited. It has been with Ash for so long and would see Misty as part of the family. It might play more with Misty's water - type Pokémon. Misty's Starmie could be very protective of the new family unit and use its psychic powers to help around the house sometimes.
Misty would probably be shocked at first. After all, she and Ash have been friends for so long. But then she would be overjoyed. She might blush deeply and have tears of happiness in her eyes as she says yes.