As others have said, a staying warm starts with a comforter that insulates well. (Real down is the best at this.) A heated blanket is not the best approach. Heated blankets aren't designed for a full night of use. They are not made of material that keeps the heat in and as you have noticed, they shut off after a few hours. This is understandable. If the blankets got really hot, insulated well and didn't shut off, they would be a huge fire hazard.

After getting a down comforter, I second the recommendation for a heated mattress pad rather than a heated blanket. Heated mattress pads provide more evenly-distributed warmth because they won't bunch up like blankets. They keep the heat under a layer of insulation provided by your blankets so they maintain heat better. Most of them have 8-12 hour auto-shutoff.

I have used and like the SoftHeat brand. In any case, you should focus your search on low-voltage mattress pads. Low-voltage mattress pads reduce the risk of fire and electrocution. They have the additional benefit using much thinner wires. Unlike with normal voltage pads, you won't feel the wires when you lie on top of it. A low-voltage mattress pad won't immediately make the mattress hotter to the touch, but that heat accumulates in your mattress and comforter. Even after a minute, it is noticeably cozier under the blankets than without one.

In summary, start with a comforter filled with real down so you don't lose as much body heat to the air. (Down-alternative make feel similar, but it does not insulate nearly as well!) Use a low-voltage electric mattress pad to provide add a gentle, cozy warmth, which is all the extra heat you need under a good down comforter. The combination of the two will keep your friend toasty all night.