I don't get it.
Why should he die from a heart attack?
Pure Diacethylmorphine (as explicitly stated) is not damaging to the body (like really toxic) - addictive of course and does stuff with your psych over time, but it's not like you get a heart attack because of
In case of an overdose suffocation would be the more probable thing
But as with all things, that's depending on dosage