Hypothermia is an emergency condition and must be treated immediately to prevent death. The first symptoms begin in the form of shivering when the body core temperature drops below 95° F. Shivering is your body's attempt to warm itself by involuntary muscular contractions.
As this condition progresses, the shivering stops when core temperatures drop below 90° F. When this happens the victim may not be able to walk, may have some mental confusion, and will eventually become unconscious. His or her pupils may dilate, and pulse and breathing may become difficult to detect.
Often, in extreme cases, people may think they are hot and start removing layers, and victims lost in cold weather have actually been found dead with most of their clothes off.
Any consumption of alcohol further increases your chances of getting hypothermia, so don't drink alcohol if you're going to be in cold weather. However staying hydrated with non-alcoholic beverages will actually help you stay warm.
Hypothermia can be a subtle stalker, as it can attack in relatively mild damp conditions of around 50° F., as easily as in real cold conditions.
OK, now for the treatment: Treatment begins with prevention (Keeping Warm in Cold Weather) , and also depends on severity. First of all, get out of the wind and wet, then remove any wet clothing and replace them with warm, dry clothes. This is usually sufficient for people with mild symptoms.
In more extreme cases, you need to start a more drastic re-warming process. If possible, get the person to a hospital promptly for this. But, if this is not possible, then begin with putting hot water bottles next to the skin in the person's arm pits and groin. You can use your regular water bottles, filled with warm water, for this.
Also it may be necessary to put the victim in a sleeping bag with a warm member of the group. If this is done, the two need to strip down to at least their base layer, and maybe even to skin, especially from the waist up, so as to get the warm body in close contact with the victim. I know this can be a little awkward to some, but in emergency situations you sometimes gotta do what you gotta do to save someone's life.
You also need to handle the victim very gently to avoid sending a shot of cold surface blood back to the core, causing re-warming shock. The bottom line is to get the victim evacuated and to a hospital ASAP!