TL;DR:

- The girl had a really rare genetic disorder, traceable to a single-base mutation, that result in intellectual disability. Her case was quite mild, she was verbal and only had a learning disability, other cases are often far more severe.

- They went for using adeno-associated virus as vector to deliver a CRISPR payload. It caused kidney and liver damage (AFAIUK due to immune response, not the virus itself? So hard to predict from an animal model.) which turned out fatal.

- The family paid a significant share of the research funding and some off-record financial favors to the research team.

- The research team's recent publication in Nature didn't mention the case at all (they basically chose keep silent about failures).

The whole story has quite some Flowers for Algernon vibes except real life is way more cynical and sad. And I disliked the book back when I read it because it felt like a weepie just for the sake of weeping.