So it's not night that is a problem, but day? For night, I can't imagine she's that cold in those temps, especially if she already has an electric mattress pad. Some thicker PJs and a thicker additional blanket should take care of that.
Does she have good socks and slippers to wear? If she is cold but walking around barefoot or in thinner clothes, this can be fixed by a rob and wool socks. If she's not willing to wear those, she's clearly not that cold.
If she's just sitting in her room studying or on the internet, she can throw a blanket over her lap. But a microwavable neck pillow filled with beans or grains (easy to make at home!) would work.
For non-bed time, I'd likely hang a quilt or some other very thick fabric time (thicker than even very thick curtains, like a comforter) and make it so that it covers not just the window, but at least 6 inches past the window. Better still if you can tack it to the wall to better seal it off. (You can pretty eaily hang a set of ropes from the top so that she can let in natural light by lifting it from the bottom and tying of the cords to hold up the bunched blanket.)
Not as cheap long term, but a radiator-style heater is pretty safe and effective. But really, I think this is a "child, learn to adapt because this is not at all unreasonable" kind of situation.