So! It took some sleuthing, but it turns out this actually is an existing version of the myth!

I'm familiar with it! It's just such a downer—and obscure—stinger ending that I am sincerely curious about the author's reasons for using it: what it's doing in the novel, literally in the sense of how it works into or against the overall argument or atmosphere of the book. Even Renault doesn't touch it and The Bull from the Sea is mostly full of the worst possible versions of its myths, which is one of the reasons I dislike it so much. Does Saint talk at all about why she retold the stories in the fashion she did?

[edit] this implies that Ariadne is involved in an actual battle at Argos, rather than merely Being Present When Shit Goes Down.

Totally agreed that not giving Ariadne a fight scene is a missed opportunity here.

(I remember the explanation at one point that he hates heroes because of how Theseus abandoned Ariadne, but I don't think there's a reference to Perseus actually killing her?? I feel like I would have remembered if it was??)

I feel especially in a children's series, that would have stood out.