I don't know whether putting the mylar radiant/vapor barrier outside or inside a sleeping bag would be best. I suspect the conditions would influence that decision. If the hypothermic person is wet, it might be better to put the mylar blanket inside the sleeping bag to protect the insulation from becoming damp. On the other hand, if the sleeping bag has a very light and breathable shell fabric and it's very windy, it might be better to put the mylar on the outside.
In any case, I think there is one important detail that is often overlooked when people use radiant barriers (like space blankets, etc.). The aluminized surface has not only a high reflectivity to IR wavelengths emitted by a person (in the neighborhood of 10 microns), but also a low emissivity. If the mylar goes inside the sleeping bag, near the skin, the aluminized side of the blanket should face OUT. In this case the blanket itself is relatively warm and it's job is to reduce IR emission. If it were turned around, aluminized surface facing in, the blanket itself would be just as warm and IR emission from the non-aluminized surface would be much greater. IR emissivity of most plastics (including mylar) is nearly 1. Putting a radiant barrier against your skin with the aluminum surface facing you reduces evaporative heat loss but not emission of radiant heat. It would be about the same as one layer of clear cellophane (the aluminum surface does nothing for you in this case).
If the mylar blanket goes outside the sleeping bag, the aluminized surface should face IN. In this case, in contrast to the first example, the mylar blanket is cold. Its job in this case is to reflect radiant heat emitted by the person. If it were turned around, aluminum side out, the blanket would still be cold, so very little would be achieved by the low emissivity of the outward-facing aluminum surface, and the inner surface, facing the person, would have a much higher IR absorbance.
These guidelines hold for use of radiant barriers in any situation, emergency or no. Radiant barriers that are aluminized on both sides are ideal because they work in almost any configuration.