It wouldn't be to stop the leaking gas, it would be to swell the rubber that surrounds the gasket of the intake manifold, (its a metal gasket with rubber that surrounds the opening of the water jackets and the fuel jackets) If the intake manifold was removed and the gasket was not seated properly when reinstalled, or the manifold was not torqued in the right pattern it may have created a gap between the water jackets and the fuel intake, thus leading gasoline to have a possible way into the coolant. Since he recently did some head work on the pathfinder, I would tend to think it may be the source of the smell in the heater.
You wouldnt think the heater core would get hot enough to ignite something, but it has happened before. I have seen numerous pathfinders that have caught on fire from leaves and other obstructions cloging the heater core. Now just imagine if a tiny amount of gasoline was running thru those lines.
Just a thought, I am dealing with a pathfinder right not that is dumping out a gallon of gas every 5 mins from the radiator, all because of an defective gasket.