Mine stays outside all winter, but I rarely get temps under 45F. If you REALLY have to put it outside, let it get morning sun, instead of afternoon sun, but like someone said before, leave it inside or outside, but don't move it around.
Ficus move so much energy, that if you move it outside, it will drop its leaves. Not because it is stressed, but because the leaves it grew indoors aren't as efficient for outside lighting. Thing is, a ficus will drop all the leaves and grow new ones, instead of making do with the old ones. If your tree is struggling, or not too healthy, it could stress it out.
Leave it indoors. Don't worry about the humidity, as long as you don't keep it near a vent or fan. Keep it watered, and don't use water that is too cold. My tiger barks and green island do fine with water from the hose in the winter, but I also keep them outside all year. If yours stays inside, you can use tap water, just let it get to room temperature so you don't shock the tree too much.
You'll find that with decent care, ficus are actually hard to kill.
Best of luck!