I think she has a good point. Though one could argue that something irreversible could happen because of the character’s haircut, the haircut in and of itself is not an important, irreversible change.

It occurs to me, on the other hand, that it’s possible for even irreversible change to be rendered meaningless. Even if there’s foreshadowing and build-up to a supposedly significant, irreversible change, if the change happens and doesn’t carry the weight implied by the foreshadowing, the reader is disappointed and bored. The reader may not understand why they’re left feeling dissatisfied, but the author failed to supply the promised payoff.

In the books leading up to Breaking Dawn by Stephenie Meyer, we are told time and again that becoming a vampire is life-altering, it’s tragic, and that it’s difficult—nearly impossible—to control the vampiric bloodlust. The question of damnation in the afterlife is brought up directly and indirectly. Bella begs to be changed into a vampire, and Edward refuses her every time, not wishing the unlife of a vampire on someone he loves. Finally, in Breaking Dawn, Bella is given her wish, but instead of watching her struggle to transition into the life of a vampire with her respect for human life intact, we learn that being a vampire is easy! It’s even fun! She has the special powers, and she has super-resistance to that pesky bloodlust after all! The issue of damnation is never brought up again, and Bella is even able to maintain a relationship with her father.

This is an irreversible change that has no meaning. Life is barely different for Bella than if she’d never become a vampire at all, except that she has super powers that are only helpful. There are no drawbacks. Edward’s reluctance to change her in previous books now seems like mealy-mouthed excuses. What was he so worried about?

So it’s not only that the change needs to be irreversible. It also needs to create conflict, and it needs to be followed through with just the right amount of payoff.