They probably word refer to themselves as a junkie - if you want to be technical about things, he was opiate dependent. If you knew anything about drug dependence you would realise that opiate dependence is hugely more problematic than using cannabis.
Heroin is rarely used alone and is likely to be used with cannabis or alcohol. It definitely causes severe problems with attention, concentration, perception and memory - and even if you did notice something on heroin, you would be unlikely to be puzzled or curious - and would be extremely unlikely to be able to remember detailed information (6 days at the very least or up to a year later). When working in a detox unit I used to review people on admission and they would have no memory of even meeting me when I saw them again a few days later.
I would also not be surprised if a 50-something homeless man with a history of daily heroin use and likely alcohol problems, had some cognitive impairment. I would love to have seen a result of an addenbrookes cognitive assessment - I did this regularly on older opiate/alcohol dependent patients and they often had memory problems.
Opiate dependent patients are one of the most difficult groups of people to work with - mainly because they can be incredibly manipulative and dishonest and will do anything and tell any lie to feed their addiction or to allow themselves to continue taking drugs.
Cannabis is a very different drug - and although it can affect memory (possibly why Amanda and Raffaele were so rubbish on the details), it doesn't cause dependence in the same way as heroin and many people smoke it socially (or even daily) for a few years as teenagers/young adults/students with little long term problems. In rare cases it has been linked to psychosis/schizophrenia (usually in youger teenagers or very heavy cannabis smokers - but it is not linked with vioence/aggression. If there had been evidence of a party at the flat, with lots of alcohol and amphetamines and the murder happened in the early hours of the morning - a sex game gone wrong could just possibly be feasible. However, for a fairly average couple of young people with no history of violence to become knife wielding sex maniacs in about half an hour, would need far more damning evidence than the foggy recollections of an opiate dependent homeless man who obviously had some mental health problems.
Regardless of whether he was a heroin user, eye witnesses are often fairly rubbish. If he had picked them out of a line-up without seeing their pictures in the news, then maybe he would have some credibility.
Here is a link on opiate dependence - note that one of the complications is memory and cognitive impairment
http://www.patient.co.uk/doctor/Opioid-Abuse-and-Dependence.htm
