I would trust the plumber.
They are licensed, which takes 8,000 hours before they can get their card, have to pass a State test, and have education credits.
Sewer gas in the attic? Is your attic ventilated? Mine is.
But never mind, plumbing vents extend through the roof so that the smells don't get trapped.
You could put insulation over the pipe to trap the heat in.
It's a lot warmer "in" the attic then it is outside.
He could also find a sink or lav inside the house, cut the wall and add a trap and funnel for the condensate.