Many many people die of pneumonia. Maybe he had more complicated problems than that, maybe he didn't... Maybe it was a strain of bacteria that was antibiotic resistant. Maybe he was immunosupressed. Depression causes immunosuppresion. However, since you were told this hush hush and aren't really close friends anymore... I really don't think that it matters how he died, it's still a terrible thing and isn't really any of your business... I don't see how it's face saving to tell people that he died of alzheimers or any reason to lie about that...

My sister in law was in a fire as a child and her lungs were severely damaged/burned. She ends up with walking pneumonia several times a year, and I'm quite certain that at whatever point she dies, she'll have pneumonia. And it may very well be when she's young. And she looks healthy to the average person, without knowing her history as a child it doesn't make sense that she always has pneumonia. And she lives in north america and has regular medical care, gets the standard treatments for pneumonia, she just can't clear it.

When my youngest brother died, I heard some of the rumors that were going around his school... "his family pulled the plug" or other ones going around the community and seriously... it was noone's business exactly what the details were of his medical history, and we really didn't need to recount them in great detail to any acquaintance to satisfy their morbid curiosity. And we especially did not need anyone questioning his diagnosis or anything like that.