Regardless of whether any beatings occurred, when someone is imprisoned in your country, they are under your care, and you are responsible for them. It doesn’t matter whether it’s malnutrition, beatings, or if he just smacked his head against the wall on his own accord really hard: it is the responsibility of the custodian to ensure the good and proper health of those in their custody. That is the position of international human rights law. It doesn’t even matter if a prisoner decides to kill themselves, a death in custody is always the responsibility of the custodian and in that case it’d still be the custodian’s fault for allowing the creation of circumstances that allows prisoners to kill themselves.

It is not usual for a person to suddenly become comatose while in custody. And even if that did happen, it was North Korea’s positive responsibility to notify consular officials and the family of that, and deliver proper medical care to them.