@nukodesu
Well...she did know that Ichi was in a relationship when they dated years ago, and still continue to date him regardless (and now she fell guilty because someone attempted suicide because of her).
But yes, it's nosense to blame only one side of the affair, two person are needed to do that, and they are equally guilty if they know the other one is already in a relationship with someone else.
But obviously, from a father, ichi's father, you can't expect him to be impartial o reasonable.