fogging up

I have a friend who has a problem with the windows in her car fogging up. Unless she keeps the defrost pretty much wide open, she's had it back to the dealer and all they can come up with is it must be wet floor mats or something she's doing. it doesn't do it when it's warm out just when it starts to cool down in the evenings when the temperature starts to change. this time of year the windows will get like a coat of frost on them if she leaves it outside