If the trap is necessary and there is potential for freezing, heat tracing and then insulating the trap may be a solution. That is commonly done in low-temp refrigeration. A 120V heat trace pulling about an amp could coexist on the furnace circuit.
Hopefully you guys can talk it through. I'd be curious to see the setup and happy to provide my 2 cents. My expertise though is limited to a milder climate so my thoughts may not be applicable.
Yeh I mentioned heat taping it. If he can eliminate any trap/ trapping in the furnace drain, this would simply allow "a little bit" of warm combustion gas out the drain, which would help keep it warm. Heat tape may well be one way out tho
