If you blow up the shot and look closely, you can tell the single light source is an umbrella. Actually it is my 60" shoot through. A large light source like this gives larger catch lights, and is enough light to wrap around her head and act almost like a weak hair light. I also used a bounce card camera left only 2 feet from her face (umbrella was about 5 feet). The white bounce card prevented shadows on her left side, but MORE IMPORTANTLY it light the hair on HER right shoulder keeping it from blending into the dark background. A separation light from behind would work too (but a bounce is far cheaper!!!!!)
POST: Skin tone color adjustment, increased blacks in RAW, slight crop.