If my friend was having a drug reaction, I would call 9-1-1, and if they recovered I would refer them to treatment options. But a judge might take care of that for me.
For the park rapist, I would raise awareness of the situation, and encourage people to walk in groups. I may even offer myself as an escort to those who want one.
If someone attempted to mug me without showing his weapon, in this case a knife, it would depend on our position relative to eachother. I'd run if I could, and attack if that wasn't an option. Either way I'd draw attention by making as much noise as possible, even if that meant screaming, but I have other options for noisemaking.