What is the ruling on not visiting a sick person who later died, knowing of his illness, and that a colleague visited him and mentioned that he was in good condition, and that the nurse was not questioned by the patient's family nor by the hospital? Is this considered involuntary manslaughter or negligence, and is expiation and fasting for two months obligatory, or is this merely a delusion?

What you have mentioned entails your performing your work in the most complete manner. Therefore, there is no sin upon you for the death of the patient, nor is there any expiation; for the basic principle is the acquittal of responsibility until an obligation is proven. And doubt about causing the death does not necessitate anything, as Ibn Hazm mentioned: "Properties are sacrosanct except with certainty." And for everything that is doubted, the basic principle is the acquittal of responsibility from it. This appears to be mere whisperings, so disregard them.