Well, you are both right. (Or both wrong.)

Fire is critical if you can't make more heat than you are losing. When you are hypothermic, your body isn't able to keep your body warm any more on it's own. You get wet, with wind or it is just cold, this can happen pretty easily.

If you are just cold, then yes, even just getting insulation under you and a windbreak up can do as much as a fire can.