Did it fall off in front or in back? How many speeds is the bike, i.e. how many cassette cogs are there in back and how many crank chainrings are in front? Or does it have an internally shifted hub?
It sounds like your husband got the chain back on properly. However, you might want to take the bike to a LBS to checked over - the drivetrain may need adjustments so that the chain doesn't fall off in the future.