That really seems incredibly high.
If you got thermostatic valves on the other radiators the relative could have a higher temp in their room, but it would achieve the same effect if you just manually turn the other radiators off. Get a second thermometer for somewhere else in the house to check you don't let the rest get too cold.
You can get a heated gilet rather than blanket, or small heated pad, or a microwaveable pad that will have no wires.