If your friend is staying at least a week, I can recommend the Chelsea Cloisters, in Chelsea, a few minutes walk from the South Kensington tube. Good location with nearby grocery stores and restaurants, and close to Kings Road shopping. I stayed there with my wife, father, and baby.

These are apartments, so they lease at a week minimum. But we enjoyed having a small kitchen handy (with washer/dryer) and the space of two bedrooms. Note, however, that as apartments, they take a deposit before you arrive and refund it sometime after you leave. http://www.chelsea-cloisters.co.uk/

Regards, Bryan Oakland, CA