It was the fact that heroin is illegal and thus street heroin is of variable strength, combined with your friend not being careful and testing his product before shooting up, and the statistical likelihood that he had other downers in his system, that killed him.
The vast majority of heroin overdoses are caused by combining it with alcohol, benzodiazepines or other depressant drugs, as these compound the respiratory depression, the rest are split between speedballing and not testing the strength relative to tolerance before having a full shot (this last one usually gets people who relapse after a period of abstinence and don't realize their tolerance has gone down). If an addict is careful there's no real reason to OD except for some freak accident like getting a fentanyl cut which wasn't evenly distributed, in which case even the smallest dose can kill if they get the wrong part of the batch. But that's pretty rare.
Of course when it comes to needles there's also the possibility of infection if they don't bother with proper IV hygiene - AIDs, endocarditis being the main two as far as actually killing goes iirc (although obviously not straight away, at least for AIDs, not sure if endocarditis can kill instantly, where did our resident doctor go?). But that shit is pretty rare these days everywhere except America & the third world, because most countries have extensive needle/swap/filter distribution programs.
But yeah, the others are right, H (or opiates in general) doesn't fuck the body up nearly as much as meth, which damages the cardiovascular system, the kidneys and the brain - although even then, it can be invisible (a friend of mine was addicted to meth and coke for over a decade while maintaining a corporate job, and you wouldn't have picked him for an addict unless you noticed his pupils), the ''faces of meth'' look is more because meth users are far less prone to good hygiene and healthcare due to the mental health issues it causes and the pattern of binging and crashing than it is to the effects of the drug on the body. By contrast, opiate addicts maintain a regular sleep cycle and tend to think far more coherently and thus take better care of themselves.
The only major physical damage opiates cause inherently is fuck with hormone production (eg. they can basically halt the production of testosterone, although this effect reverses, at least mostly, once the opiate use stops). Of course that's excluding damage from improper needle use, and I'm not sure if smoking it damages the lungs (although my guess is not significantly, or we'd know about it).
To the guy who said he got tinnitus, my guess is you were taking pills combined with APAP or another NSAID - it's been a while since I did any reading on this (I'm boring and mostly sober now) but I'm almost certain it's the APAP/NSAIDs which cause the tinnitus, not the opiates themselves.
/druggeek