What camera are you using? If I'm not mistaken most all of Nikon's DSLR cameras have a small pop up flash that should be adequate for getting the shot. Even then getting a black background in a small apartment will be problematic without a black backdrop of some type. Is there some requirement that it be in her apartment? Without flash or a lens with an f/2.8 or less is probably more of a problem than it's worth. In the conditions you describe to get good natural light pictures you should use a lens with an f/stop of 1.8 to 2.8, with the f/1.8 being the preference. Nikon makes two lenses that fit that range especially for shooting in small apartments with not much shooting room. They are the 35mm, f.1.8G, AF-S and the 50mm, f/1.8D AF (Both lenses are DX). Of these two the 35mm is a little more expensive but in a small apartment is probably your best bet. Nikon also makes these two lenses in a f/1.4 but are considerably more expensive. You don't say what camera body you are using, I assumed it's a DX type. If it's an FX, 35mm size sensor, then the lens choice will be entirely different. If you would please complete your profile with camera and lens information and we can give you more information on your question.