1: Necking too much or something nasty
If the person is freaked, get them somewhere where they can’t hurt themselves (try and avoid places with large glass windows and that sort of thing). Also, try and find somewhere quiet and cool, talk in a quiet voice and reassure him or her.
If the person is unconscious, learn the recovery position, its easy enough. the point of this is that if the patient is sick, they won’t choke and die. This condition is very normal with ket heads, beer monsters and brew crew in particular and there's not a lot more you can do.
One thing to look for is white powder around the nose. This is almost certainly Ketamine and the person is probably in la-la land, the "K-hole". They should come around in about 10 - 20 minutes.
If there's a friend nearby, try to find out what the person has taken. If there's no obvious reason for the situation, or if it's clearly a dangerous overdose, maybe an ambulance is needed. If need be they can be moved.