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.
He would probably try to find out the reason behind Lillie's betrayal. Ash is not the type to give up easily, especially when it comes to his friends. He might start investigating, talking to other characters who know Lillie, and trying to piece together what made her turn against him. This could lead to a journey of self - discovery for Ash as well, as he questions his own judgment in trusting her.
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.
It could be that Lillie betrays Ash because she believes it's the only way to achieve a greater good in the Pokémon world. She might think that Ash's methods are too dangerous or not in line with what she believes is right for the future of Pokémon and humans alike. This would create a complex conflict between their ideologies.
I'm not sure specifically as there could be many different takes in fanfiction. It might involve scenarios where Ash and Lillie break someone's trust, perhaps in the context of their adventures or relationships within the fan - created story.
Ash would probably be devastated. He would feel like his world has been turned upside down. At first, he might not believe it and think it's all a big mistake. As the reality sinks in, he could start to isolate himself from others, even from his Pokemon for a while. However, his strong will would eventually kick in. He would use this as motivation to become stronger, not just in his Pokemon battles but also in his ability to face challenges alone, without relying on his so - called friends.
Ash could get angry. He has a strong sense of justice, and he might feel that May has violated their friendship. He could challenge her, not in a violent way, but more in a way to get an explanation and to try to make her see the error of her ways. He would likely be determined to show her that what she did was wrong.
Ash would likely be shocked at first. He's so used to seeing the good in people, especially fellow trainers. But then he'd probably get angry and determined to set things right. He has a strong sense of justice, so he wouldn't let the betrayal go unpunished.
Ash would be extremely devastated. Pikachu is not just his Pokémon but also his closest friend. He would likely first try to reason with Pikachu, not believing that Pikachu could truly betray him. He might be in denial for a while.
I'm not aware of any particularly popular ones off - hand. Fanfiction is a vast world, and while there might be some great 'ash and lillie betrayed' stories out there, it's hard to single out popular ones without doing a more in - depth search.
Ash would probably be initially shocked. He is used to seeing the good in people and would not expect such a betrayal from Cynthia. He might stand there in disbelief for a moment, unable to process what has just happened.