As indoor air is consumed by a furnace, replacement air must come from somewhere. You can allow the furnace to inhale indoor air instead of piping it in and the lower part of the house will depressurize enough to draw in makeup air from the paths of least resistance. This will cause cold air infiltration in places they probably do not want it. As cold air enters the living space, it will cause a call for heat and run the furnace anyway. Piping in the combustion air reduces cold air infiltration while reducing the chance of backdrafting any remaining atmospherically vented appliances.
Tell him to pipe in the air then spend his money on sealing the ducts and weatherizing the home. These unpiped Cat.IVs can be dangerous.
Bob

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Keep the fire inside the fireplace!
IAFCI Regional Director & Certified Fireplace Inspector
Certified Master Hearth Professional & HPBA BoD