OK, I wouldn't be so quick to leave the casualty to die. First I'd want to know what sort of substance we are likely to be dealing with. The location and timing of the event are going to be a good indication, and of course the person that called it in as a Hazmat is going to have some idea...
Is it farmer Joe unloading some herbicide from his ute right next to his weed sprayer, or is it a postman who accidentally opened John Howard's mail?
In the situation where you have no idea what it could be (like a 'mixed dangerous goods' truck), option 2 would be best, but if you can be "almost certain" that its a farm chemical, and you saw him move, option 1 might be worthwhile...