Maybe you could ask your FIL to back up his claims that they are biodegradable as he could have access to 'inside infomation'.
Given the right conditions, they probably eventually break down (as people have said not in landfill, in plastic and in an airless environement) as I believe if something is airless not even something which is bio degradable / compostable will disappear. There is also something about the chemicals in the absorbant inner that could contaminate the earth / ground water especially depending on whether the nappy was just wet or soiled.
Leading on from this does anyone know how long it should take something that is biodegradable to disappear? - I'm thinking there should be some sort of year(s) limit otherwise everything would be degradable over time - just think of how a car's metal rusts to nothing, but we don't call those biodegradable!