I only do executive and general business portraits now. Her professional head shot isn't just about lighting. It is a reflection of her--her style, her profession, her business essence. A professional will interview her, advise on her clothing choices (for color shots, she needs to avoid certain colors; for B&W some colors come off better than others.) It can be done with a P&S and its built-in flash, you just need to redirect the light off of a white reflector (or wall) while incorporating the window light, determining which makes better key and fill. If the P&S has a zoom feature, use it. It will help throw the background out of focus.
My last head shot for a magazine article was done outside with a Olympus EP1 P&S. I had the aperture open and the lens zoomed out to eliminate the busy background. I used the sunlight bouncing off of the white walls of the house to provide a rim light, while the 4 o'clock sun was my key light. Granted, it wasn't a formal head shot (the magazine is a rural oriented publication), for that the style of dress and the background would have been different, but it proves it can be done with a inexpensive camera and by modifying available light sources. If I had a suit and tie--and hung a backdrop in back--it would have looked just like a studio business portrait.