@ punkie
Don't worry too much about the siren. There are many reasons why they could have turned it off. Kenny mentioned the main ones, but there are two more:
1) There may be no place for the traffic to give way, so having the siren on may be counter-productive.
2) They might need it quiet for a bit, so they can hear something faint. I've been in there and it is quite loud with the siren on.
As for whether you could have done something: probably not. If the guy took some pills, you have to consider something:
1) He may have taken those pills to treat an existing condition which, if serious, may not be remedied on the pavement.
2) He may have taken pills that are not related to any condition. This could be mistaken medication or overdosage.
In both instances you have to know what the medication is before you can decide whether you're dealing with (1) or (2) above. And there isn't much you could do. It's different if someone chokes on a meatball at a restaurant and you can't do the Heimlich Manouevre, but your man is a difficult case for a by-stander. It needs a doctor. I don't think I could have made it better for him, because I don't know enough about medications.
Anyway, I tried to save a young woman of 26 in the hospital when there were no doctors around for the first few minutes. I started CPR, and when the doctors arrived they told me I was doing a good job and I should continue. I went for a good ten minutes but it was no use. She was gone. Internal bleeding post-operatively (she had a bad pelvic fracture). I think the guys already knew it because her tongue was white and she had no dolls-eye reflex, but something had to be seen to be done and it was good experience for me, from a technical point of view.