If you're out in the cold and one of your companions begins to shiver violently and then becomes lethargic and drowsy, move the person to shelter out of the wind and rain. Remove his wet clothes and replace them with dry ones. Cover him with blankets or get him into a sleeping bag. Call for medical help, but do not leave the victim alone.
Watch the victim's breathing and be ready to give artificial respiration if necessary.
If he's fairly alert, feed him sweets or chocolate and, if available, warm, sweet, non-alcoholic liquids.
See also: Frostbite