What caused my client’s abusive husband’s passing in the hospital, as he was there to have knee repair surgery, not because of a mortal illness?

As you saw intuitively, he was the victim of a medical error that ended his life. So this is an irony of circumstance where there was a coupling of the Law of Karma and mischief engineered in the manipulation of medical providers to be slipshod and put him, as a patient, at risk, and that ended in his tragic death. We say tragic because it was unnatural and short-circuited lessons underway. Even though in some ways it is a blessing for his surviving spouse to be free of fear, because he was never willing to let her go and have peace and friendship with another life partner, it is worse for him to end this way because it will add to his karmic woes personally, and that will require even more healing in the future to work his way back to any semblance of a normal life, because he was truly living as a psychopath, leaving extensive damage in his wake for all he encountered. This became a karmic match to those in the healthcare system who, karmically, are in arrears as well, because what they do is so often harmful, although not intended. But the Law of Karma is dispassionate, so harm caused others will harm the self and they become perpetrators as a consequence of all of the downside of medical treatments they provide as caregivers. This developed an alignment of potentials, both for the husband to meet with a karmic reckoning, and for the healthcare workers to be vulnerable to manipulation and make mistakes to compound the dangers of their healthcare measures when administered inappropriately, and that conspired to bring about this dark chapter for all involved directly.