I will not disagree with you here. She is being very harsh in her revenge, and perhaps she feels it is justified, as he has shown little, if no, remorse, to her view of it. And while it may seem over the top, and overstepping on her part, we wrote it like that on purpose. (Again, this is the Netorare path, and we wrote these scenes for that reason. She is cucking Fred, and doing it to make him jealous and humiliated for how he treated her.)
We have stated from Day 1 of this game, that cheating on Mary would have consequences, and now, we are seeing those consequences. If you, as the player, don't like those consequences, then don't cheat on Mary. That is the point we were making.