I'm sorry but that sounds painful
I'm sorry to hear about your friend. As for the cause of death, usually with strangulation it's because the blood circulation to the brain is cutt off, rather than asphyxiation. Your friend would have passed out fairly quickly before he died so he wouldn't have suffered for long. Still, it's a horrible way to die. I hope the bastard who did it has been caught.
Either way, it would have happened in a matter of minutes. Not exactly slow but not exactly fast. Sorry, I'm not trying to be a downer.
I don't think anyone can really answer that, but its horrible and I'm very sorry.