Which advice would you give to someone who has just been diagnosed with Parry-Romberg syndrome / Progressive hemifacial atrophy?

I would first say find a great doctor that will listen and researcher.

First, I'd give you a hug and say this sucks, I know. Hang in there. Life can still be good even if you hate looking in the mirror or seeing yourself in pictures. And let yourself be photographed. What your friends and loved ones see in pictures is the same image as what they see in real life. It's only shocking to us because what we see in a picture is the opposite image of what we see in our mirror every day.
And then I'd urge you to seek treatment. Dr. John Siebert at the University of Wisconsin in Madison (USA) is a world renowned expert. He's the head of Microvascular Plastic/Reconstruction Surgery. He's done over 140 Reconstruction surgeries on PRS patients. His experience is that, even when the disease is active, putting healthy tissue in the diseased site actually stops the progression. Waiting till the PRS aquiesses--as some doctors advocate--is fruitless. Fat injections are also ineffective as it all just gets reabsorbed.