People die from weed. Lung cancer for instance. You think weed is not bad your heart?
There have been precancerous cells found in heavy marijuana smokers, never a case of lung cancer that could be attributed soley to marijuana smoking however.
Yes people do die from weed, but like you said he was fat you have to smoke like half your bodyweight in weed in a few hours to overdose, so lets say he was 300(or more) he would have to smoke 150 pounds over a few hours i highly doubt it was the weed....
Even the government admits there is no such thing as marijuana overdose. Try 1500 (that is THOUSAND) pounds in 15 minutes.