What I've seen claimed is not that she was a full ghostwriter, but that that she did a lot of the research of the non-horse-related stuff, he wrote them, and she edited them. (Wikipedia, mostly quoting from interviews with them.) Whether "editing" means "rewrote so extensively it was basically ghostwriting," though, I cannot say. At any rate it seems to have been fundamentally a team effort between them, and acknowledged as such. And yes, I totally agree about the awesome women!