Is there some kind of law or ordinance where they live that requires such a standard to be met for a house to be sold? If not, this is just a crafty home buyer getting someone to use standards that do not apply in order to leverage your parents to fix a potential problem on their dime.
They may want to do it if they need to sell the house and there are no other buyers. Or, maybe they have other prospects and can tell the buyer to fix it himself.
Again, if there is a legal requirement to meet this standard it would be another story. My concern would be that these buyers are just at the beginning of a whole lot of more ways to get things fixed or money knocked off.