Betty - the vegan girlfriend

It's usually annoying when they change plots and characters of books when they make thwm into films in order to make them more attractive to viewrers. But if they had to do that, writers could try to keep more of the book spirit. I mean if they wanted to put a love interest in the film, why didn't they make Betty more like Belle, James's friend in the book? She was cool and not boring and understood James's struggle to get clean. And why do they need to depict a vegan as an alternative person like a modern day hippie? There are are doctors, executives, professors, scientists who happen to be vegan. And I wonder if the screenwriter has ever tasted a vegan meal!