When you meet your ex-boss for lunch, do you greet each other with a hug?
I'm all pro-hugging, which have resulted in one or two (or many) uncomfortable situations where people probably felt I crossed the border when they would have preferred a handshake.
So if your ex-boss is a high-flyer audit partner, and hugging clients or colleagues is an alien behaviour, then my lunch-greeting hug is probably as uncomfortable as wearing a pair of too small shoes. You can do it and they look alright (or even really good), but the feeling of discomfort is always there.
My philosophy is 'one hug too many can't hurt'. So I hugged the audit partner. Not once, but twice.