There is a product available made to limit lens fog. It comes in a small spray bottle. Motorcycle shops commonly have it near their helmets or at the checkout counter.
He should not call this a “gas mask”, it would be tragic for someone to die from inhaling vapors or gas thinking this would protect them. The filters are designed for particulates. The other aspect he doesn’t cover well is the seal between the bottle and the filter. That seal must be perfect or the mask is useless against any harmful contaminants. It would be worth while to tape the edge of the filter to the bottle. These would not be part of my preps in an attempt to save money, more like a last desperate attempt to survive something I wasn’t prepared to face.