So I would like one of my major characters to start out somewhat good friends with my protagonist, but then he decides he doesn’t like the side the protagonist is on, and switches to the side of the antagonist. (He sees the flaws in the group the protagonist joins, and hasn’t yet seen the flaws of the other side). He later realizes that the side he is now on is not the side he wants to be on, and would then leave the antagonist’s team and go through a redemption arc to rebuild his friendship with the protagonist. I would like if he could end the story on pretty good terms with the protagonist.
My question: Is this too much for a single book? Would putting this whole arc into a single book cause the changes to feel rushed and unnatural?
I keep going back and forth with the story this is a part of in regards to if it should be a single book, or a series. The general plot might work well in both forms.