It is probably worthwhile to review the care and treatment of hypothermia victims. One good source is the
State of Alaska Cold Injuries Guidelines. These were developed by a panel of experts, including Prof Gorden Geisbrecht. These Guidelines are widely cited and have become the model for many organizations outside of Alaska EMS. The Guidelines cover all levels of treatment, including in hospitals. For most of us, the relevent parts are in the "General Points", "Classifications of level of hypothermia", and "First Responder/Emergency Medical Technician-I" sections.
Some general comments for field situations are if they are still shivering and talking more or less coherently, then they probably have only mild hypothermia. You can be rather agressive in rewarming these folks. Prevent further heat loss, rewarm with external heat if available, feed them high energy food, get them moving.
If they are no longer shivering and/or talking, then they have probably slipped into moderate or severe hypothermia.
Handle these folks very gently. Provide insulation to prevent further heat loss, and gently transport them to a hospital. Attempting to rewarm a severe hypothermia case in the field is very risky. The good news is that if you can prevent further heat loss and handle them gently, they can survive many hours of transport to a proper clinic or hospital.
The best treatment of all, of course, is to not let things deteriorate to hypothermia in the first place!