Not with your actual advice, which I do agree with, but about him being nonchalant about this. The very fact that he chose to ask the question here instead of just trying to wing it on his own should the situation come up proves that IMO.

The "normal" etiquette when running into a hooker/john is to simply "keep moving" but not all situations are normal. Jula's advice to ask her directly is good advice if you think the chances are good that you will run into her. I did that exact same thing when learning that a provider I saw from time to time hung out in many of the same places as me. When I asked her what she preferred if we ran into each other, she told me to call her by her "real" name and proceeded to give it to me.

That is one possible outcome, but like I said the "normal" etiquette is to say nothing if you run into someone from this world in public.