Historically, that is what it meant to be middle class. Before there was an established middle class, there was only the upper class - aristocracy who inherited wealth and land - and the poor. You might find a rich merchant from time to time but they were the exception and not a class. Once the industrial revolution kicked in, it became possible for a large amount of people to amass wealth without having inherited it. If you were able to do that, you were part of the new "middle class". Of course, there is often much sniffing and consternation over the super-wealthy middle class occasionally trying to elbow their way up the social ladder into the upper class.

If this woman's father bought her a trip in 1959, then he may have made his money in the 1920s, when the middle class was only just firming up. It would have been correct at the time to think of him as middle class, even though to us today he seems pretty wealthy. That's because his class status was defined by him being a businessperson who presumably did it himself, as opposed to having inherited wealth, land, or title.

The rise of the middle class was the cause of much cultural introspection at the time. There's a lot of thoughtful and beautiful literature about it, like Howards End[1] and The Forsyte Saga[2].

Of course, today "I'm middle class" means "I earn enough to own a car and take a vacation once a year." But that's a modern shift of definition and not quite what it meant for this woman's father in the first half of the century.

[1] https://standardebooks.org/ebooks/e-m-forster/howards-end

[2] https://standardebooks.org/ebooks/john-galsworthy/the-forsyt...