I know there's a popular headcanon that Grace's crew died because of feeding tubes malfunction (based on the paperwork Grace was doing right before the explosion), but — in the book he specifically says that even after the accident he kept dealing with that same paperwork on minor Hail Mary issues, so I doubt that feeding tube problem was left unaddressed. May I offer instead:
Grace was put into coma by the people who cared about him. They (especially Yáo being Yáo) probably double-triple-quadruple checked everything. They watched him sleep for those first few days — I doubt they went into coma immediately after leaving Earth's orbit. They probably talked to him, assuring him that he'll be okay.
Ilyukhina's coma procedure was probably supervised by Yáo. He made sure that everything was in order, but — he is just one man and he is not a doctor. There was much more room for mistakes.
When Yáo went to sleep, he was alone. He had to rely on the technology completely.
We know that he died first.
