Unlawful act manslaughter — Kennedy (No 2): free informed self-injection breaks the chain

A man prepares a syringe of heroin at the request of his adult friend, who is fully conscious and lucid. He hands the loaded, ready-to-use syringe to the friend. The friend takes it, injects himself, suffers a reaction and dies. The man is charged with unlawful act manslaughter. The prosecution argues that supplying and preparing the drug was an unlawful and dangerous act that caused the death. There is no suggestion the friend was pressured, deceived or lacked capacity. Which of the following is the best analysis of the man's liability for unlawful act manslaughter?