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 would be deeply hurt and in disbelief. He may initially freeze, unable to process what's happening. After that, he would probably try to understand the reason behind her actions. If he thought it was some external influence, he would fight to free his mother from it. He might also feel a sense of guilt, wondering if he had done something wrong to cause this, even if it was not his fault at all.
Ash would be very worried. He would immediately rush to Serena's side. He might stop whatever he was doing, like a Pokemon battle or exploring, and focus all his attention on her. He would probably ask if she's okay and check for any serious injuries.
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.
His friends would be extremely shocked at first. Misty might scream and be really confused about what's happening to Ash. Brock, on the other hand, would probably try to stay calm and start thinking of ways to reverse the transformation right away. They would all be worried about how Ash is feeling in his new form and if he's in any pain. They would also be concerned about how they can still communicate with him effectively, since he's becoming a Pokemon.
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.
His Pokemon might be a bit confused at first. Pikachu, for example, is used to their adventures on the road. But they'll soon adapt. They could make new Pokemon friends at the school too. And they'll be there to support Ash during his school challenges, like in battles or tests.
Ash would probably be very shocked and hurt. Pokemon are like his family, and for Misty to steal one would go against everything he stands for.
Ash might be heartbroken at first. He'd feel a deep sense of loneliness and confusion.
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.
His friends might be really confused at first. Pikachu, for example, might give Ash some really puzzled looks. If Ash tries to explain to his friends that they're all part of a fanfiction, some of them might not believe him at all. Misty could think Ash has hit his head or something and is just spouting nonsense.
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.