Probably just damp from sitting around doing nothing.
Tell him to put the windows down a bit if its in a garage and get silica beads and put them in the interior of the car they come in little bags and they absorb any moisture.
Same stuff you get when you buy new trainers but the bag is bigger
Something like this will do
http://m.ebay.co.uk/itm/171271771237?nav=SEARCH