Well it’s the dukes fault for making her find a treatment in the first place.
SPOILER ALERT!!
He's way crazier than you thought. The doc actually forced the Duke's dead twin brother to have Damien. He used the craziest way possible. (I read in the novel)
Because he's worried of her health. Women not getting menstrual period is a major health issue. Plus, the Duke doesn't know that to consume hagwort and not having menstrual period means the women can conceived the child of Taran. So it's not the Duke's fault. It's the PSYCHOPATH'S FAULT
But like she also doesn’t know and she just wants to help the duchess if she does actually ask for the treatment I just know Mr.incest is up to some bad shit
Well from the duke perspective he didn't know whenever this "diseases" gonna harm Lucia in anyway so that's way he's trying to cure it and that before Lucia told him that she actually can cure it herself
you see, if lucia dosent want the treatment and the duke doesn't want a baby why are u forcing the treatment? its getting kinda annoying