I have just read this, firstly OP I have to admire the beauty of your self composure and grace under pressure. If I had even 10% of these qualities I would be happy, except I've been called a force of nature, with a red mist descending when it comes to my emotions. I just want to put a slightly different perspective on something that seems to be overlooked.
This man, by the sound of it is at the pinnacle of his career, and when your in this position, you do need a sounding board, this other person is integral to your daily working life. Before any major decision is made, it is always prudent to discuss this with someone who understands your work and can counter your reasoning, objectively and give an unbiased opinion. They are your counsel, in actual fact they are your rock.
This new girl may be 'stunning' as you describe her, but I cannot imagine she could not comprehend for one second or she has one iota of what his work involves or can offer any constructive advice to offer him regarding his working world. The sparkly shine will soon tarnish on a relationship made purely on the grounds of attraction.
This man is not stupid he will realise this, and I believe it has already dawned on him these facts as outlined above.
Reality will hit home sooner than you think, so be prepared for him contacting you at some point.
You will keep beautiful dignity intact, and this man will realise he's made a terrible mistake. What happens then is down to you both.