Okay, so it wasn't a diet.
Though the introduction of this arc was kind of abrupt.
If the Countess seemingly now opposes her father, and what she was originally sent for, is now acting on her guilt, then perhaps...
At this point, she might be considering that she's at a point of no return, and the only thing she can do is to bring her father down along with herself. If those documents are genuine and cannot be disproved, assuming her emotions were genuine and she really came to love the Count and her stepson, she might be thinking she's in a corner where this is the only way she can make amends. By foiling her father's plot, and allowing herself to be punished for her part in it.