Use a spot meter, if you've got one, to get a general idea of the light level outside of the window (little points of light on a skyline are probably too small to measure individually, but you can get a general idea by sampling different clusters of lights). I like to use a digital SLR to check my exposure in situations like this. Once you have the proper exposure for your background, just light the actor relative to that light level. If you don't have a spot meter, you'll probably be okay if you just shoot wide open and light for that.
Hope that helps!