Actually, from a medical standpoint heroin addicts do pretty well if they have access to a accurate does of a pharmaceutical grade product, clean needles and medical care - certainly better than if one or more of these factors is absent.
Yes, and that was the case in the UK prior to the early '70s, so it's not like someone making such a statement is theorising.
The only things prohibition has done is make heroin addicts more likely to indulge in criminality to keep up their habit and more likely to overdose, be poisoned or become ill from the by-products of the illicit supply of the drug.