The previous owners of my home routed the flex duct from the bathroom fan
right to the underside of a normal roof vent and sort of wired it in place.
I'm about to re-roof the house, and although I'm tending away from this
idea, an acquaintance comes highly recommended for this kind of work. Three
person crew, all permanent people. But, while looking at the roof with him
today, he gave me a blank stare when I mentioned a vent specifically for
fans. Can anyone describe what these things look like?

Yes, I have two estimates from "real", insured companies. :-)