Use the local adjustment brush and set both clarity and sharpness to -100. Paint over the face a few times - ie you'd have a few local adjustment pins.
Of course, if you are perfectly in your rights to take the photo, stand up for them and publish the photo straight.
Then try 12 times. Sure the flow is set to 100 each time you paint, and the automask isn't on? Then try -4 exposure, -200 brightness too. But to be frank, Lightroom isn't designed for such things and you could do the task in Photoshop much more easily.