I use to smoke in my bathroom all the time... in my house I would just turn on the exhaust fan thingy that every bathroom has, and by the morning the smell would be gone... make sure and keep your door open though so the air can circulate, and if it still smells right before they get home for some reason, use a much febreeze as it takes... but 1 day is plenty of time to air out a bathroom, two would guaranteed no smell.