The father is the common "good intention that cause neglect" abuser. It's clear that, even if he probably never forced her to follow his same diet, she think that she must doing this to get his approval (and attention), and he did pass some incorrect bias, like how she say her diet is healthy. But a diet so invariable and so unbalanced cannot be healthy, even if all components are. Caloric deficit ISN'T healthy when not used for weight loss (and if excessive even for that), it will cause the body to attack your own muscle mass, lowering your metabolism (paradoxically causing an easier weight gain once you stop starving yourself), and if done for prolonged time, it can create long standing issues involving cardiac issues (multiple people are reported to have developed cardiac valve issues (some requiring repair or substitution)) after long self-starving.
Glad that at least isn't shown positively, and when the father doubled down her conviction slipped, and the maid pushed her to increase her intake (even if for the sake of performance). The father could also easily fix by suggesting a more balanced diet for her.
This is a particularly triggering for me, as I risked permanent damage for a diet. Mine wasn't even full caloric restriction, but was carbs restriction (more then half of my carbs intake removed). It was necessary, I was bordering insulin-resistance and was 120 120 kg, and it was effective. In 6 months I lost 30 kg. However my dietologist wanted to make me loose the last 10 kg (arriving to 80), and made me continue the same diet, this while never communicating the risks of this kind of diets (risks that I later discovered were well know in the literature). After a couple of months, I started to develop extreme fatigue, I become extremely exercise intolerant, my mood was swinging soo much my doctor started to think I developed BPD (luckily I discovered these could be fixed by increasing eating bananas, otherwise I could have ended medicated uselessly). Luckly I collected all evidence to convince my father, that in that period was in the "trust doctors blindly" camp (now he is more rational, after he saw firsthand how some doctors are strikengly better then others (including a new dietologist he started going to years later), while others are arrogant, aggressive, and some even incompetent, when not borderline criminal) , to make me stop the diet, so the wrestle was minimal. It took 2 or 3 months to recover my physical health as it was before starting to having problems.
So while this may be an excessive wall of text for a manga first chapter... seeing certain behaviours from someone that hold some sort of authorithy (like a father or a doctor), always disturb me deeply (even when they are fictional characters).