@Georg
Yep. Rereading my reply, I agree with "Little bit harsh" being a major understatement assuming there is no translation interference.
Now for this,
I'm not exactly sure what other thing should the father had done to be "less of a dissapointment" so she wouldn't need to "correct him"... Not tell Alberto to fuck off??
I don't know about you but telling Alberto to fk off (especially immediately) is basically my second last choice among the list of "doable things" I could come up with (last choice being ignore him and let the problem fester).
Other things I would do, not listed in any particular order include giving the guy another (secret or not) loyalty test, pretend to know nothing and spying on him/feed him false information, put him under house arrest with no birds, try to bring him over to my side again, etc. Well, there are plenty more if I think about it harder.
The real "disappointment" part is that the traitor was essentially "family/close friend" bound by a human noble mindset. It wouldn't be strange for the daughter to pick up on their relationship considering how unnaturally sharp the author made her. Yet the father, after exposed to spirit culture, immediately threw him off without bothering to see whether the guy had any troubles/issues/skewed priorities leading to such traitorous behavior. While this is mostly due to his doting father overprotective nature, it sort of shows that the man doesn't quite treasure his relationship enough. (Like how he abandoned his home for ten years)
In the end, the evaluation of the harshness of the word trash heavily depends on whether you agree/disagree with the father's actions along with the actual untranslated word used.. (iirc ヘタレ, Hetare, means Good-for-nothing, one way of translating can mean "trash" but could usually done for comedic effect on people who can't get themselves to act, less harsh than クズ Kuzu trash at least.)
Basically, I read that "trash" as Hetare or something similar which is why I am more "forgiving" with the harshness. Not saying that I "agree" with that harshness however cute though. Like I said, it is mostly manipulative. (I also think she is a sadist that loves teasing her father)