Kendra's curse is acknowledged in-text as being Disproportionate Retribution towards a spoiled child. In an Offscreen Moment of Awesome as she relates to Kyle at the end, the other witches were furious with her for being so reckless. They put a spell on her, in turn, to become Magda, his housekeeper and mother figure, and help him break the curse. If she fails, she'll be stuck as Magda forever, just as Kyle will be stuck as a beast.
Knowing this in hindsight, Kendra ordering Kyle not to smash the Magic Mirror becomes a Rewatch Bonus; it's not just meant to give him exposition but to save herself as well! She's terrified that if Kyle just gives up, and he has no reason to trust her at this point, then her chances of going home are nil.
The turning point of Kyle's Character Development is when he calls out Kendra for the curse not just because of what she did to him. He also says that it would be cruel to force an innocent person to stay with a beast for the rest of their life. Kendra is The Stoic but you can tell she is ashamed.
In the book, Kyle rushing to save Linda at the end.
Movie
Kendra stepping out of the elevator as the new assistant of Kyle's father.
The alternate ending version of the movie mirrors the book so beautifully. It's a wonder the directors bailed out on it.
