Short answer: you can't.
With film, the response curve had a gradual roll-off of sensitivity in the highlights and shadows, and unless you were several stops off from correct exposure, you could coax some kind of detail out of overexposed highlights.
With digital, this is NOT the case. Digital has a linear response to exposure, right up to the point where color channel(s) clip, and then increasing exposure further makes no difference whatsoever in the digital values output by the sensor/ADC. No matter what you do, you can't dig detail out of a featureless matrix of 255,255,255; it's mathematically impossible.
This is the foundation of the "expose to the right" doctrine when shooting digital; get as much exposure as you can, but DO NOT overexpose the highlights, or you have lost them, and cannot get them back. When you see a spike on the right side of the histogram, that is a danger sign indicating you have a problem. Either you need to reduce exposure (which runs the risk of increasing noise levels in the image) or else you need to use a multi-shot HDR technique to blend multiple frames with different exposures together to capture all of the dynamic range of the subject.
You have two options:
2: Paste an outdoor scene image into the blown-out window and try to make it look realistic and hope the client doesn't notice or care about your incompetence.