It’s because she’s angry and upset and wants to hurt him back, and this is the way she can do that. She wants to let him know how much of a piece of shit he really is, in front of people, so he can’t hide from it anymore. It hurts her, but it hurts him more.
Maybe she can make him feel a twinge of guilt or regret over what he’s done. Maybe. If not, at least she gets cake.
But for real, she’s upset and wants to hurt him back, and make sure he knows he’s not a good person, because he probably gaslit her and has been denying it for a while.