Flavoring a perception Aid Another shouldn't be as difficult as people perhaps think.
The ranger, knowing that his party won't be good at spotting stuff, asks them to not bother looking, but instead to just listen.
In the real world, focusing exclusively on one sense increases your chance of detecting something via that sense.
The perception check does not discriminate for this, so the Ranger with the high perception uses all his senses(he IS a ranger, after all). Because the ranger is covering sight/smell, the party is free to focus on one sense.
In real life, this situation would actually enhance the perception checks of the party members. The mechanics in D&D, however, are built such that the bonus would have to go to the Ranger instead, in the form of an Aid Another.