For the air you draw in, you need to draw an equal amount of air out. So you're exhausting the humid air that is in the house (from breathing, cooking, showering, etc.) and bringing in the dry air.
Once he has this under control, I doubt he'll need the exhaust fans because the air exchange on the furnace will equalize the draw in versus draw out. While he's getting rid of this too-humid air, though, he should put some fans on. I'd have the kitchen range fan on low for the next few days and keep an upstairs bathroom fan on, too.
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