Is the illness of a Muslim who died because of it, and whose body no longer had space for injections, an expiation for their sins, and will they be rewarded for it?

Diseases and calamities expiate sins, for it is stated in the Hadith (definition) that any calamity that befalls a Muslim, even a thorn, God expiates some of his sins by it. On the Day of Resurrection, people who lived in ease will wish that they had been afflicted with trials when they see the reward for them.