So her girlfriend of several years dumped her and married a guy right away? I am always quite confused when I read that kind of storyline in a manga, do people in Japan really get married that sudden?
It's hard to explain properly without touching up on the culture in places with a lot of influences from East Asian cultures. There's a lot of implied expectation for people to get married and have kids; "fulfill your filial piety".
The way marriage is seen is less of a "two people in love getting tied" and is more "two families becoming intertwined" sort of thing. So for some people, going along with the family's decision OR deciding for themselves that this other person is better for various reasons that may either involve the family that they are about to build or their relatives is a higher priority than picking the partner you already have/had.
I mean, those "marriage interviews" (I don't know if that's the right English term for it) are still very common and, unless you're using a matchmaking service, families tend to look for people that they either already have some sort of connection to or people who will bring better connections/opportunities for their family or their would be descendants.
Not saying that there's anything inherently right or wrong with the system, just that there are people who prefer to go along with that instead of going through the trouble of finding a partner themselves not knowing if things would even work out.
It's a whole thing really. It works for some and doesn't for others. Usually, the people involved pretty much decide very early on in those talks if they will get married or not.