Sorry for being a wet blanket, but working full time will always be the better choice than part time, and you shouldn't tell someone not to change for the better.
Cute chapter though.
In Japan because of Japanese labor laws part-timers actually get paid more per hour than full-timers, which is why most workplaces have a policy to limit the number of hours a part-timer can spend working in their premises. New Game! actually made this a joke at one point, admitting that the part-time bughunters can't overtime because then they'd get paid more than the actual programmers (including the head of the debugging team hiring them).
In theory a part-timer holding multiple jobs with work hours equalling a full-timer can bring in more income. The only problem is that they don't get work benefits, but that also doesn't cause that much trouble for the average person because Japan has subsidized healthcare and the Ward system. If you're thinking about salary progression, Japanese wages have been stagnant for two decades, so a full-timer can only expect better salary by getting a higher position (which is impossible for part-timers) but that itself is extremely difficult in the overcompetitive Japanese workplace. Pension is also basically gone in Japan today, and most people need to save on their own.
This is actually why there are so many NEETs in Japan. Working part-time just gives you better quality of life overall if you're living alone.
As to why the Japanese labor market works this way, it's because this kind of system is better for companies as it provides very flexible hiring system - or at least it WAS, until around last decade. Now high turnover rates and a declining population create a nightmare for Japanese service sector companies due to the need to constantly train new workers that might disappear at any moment.