Depending on how long it was like this, and how DIRTY the conditions were that he/she drove in:
dirt entering combustion chamber can slightly scratch chambers, and make the entire air tract dirty,etc causing: loss in compression, and poorer fuel mileage.
Again.. this is hard to state as fact, because of the myriad of variables there can be (conditions, maintenance, length of time, time of year.. particles in air.. etc, etc)
Clean the maf, and the TB, and hell the UIM/LIM (you will love yourself for doing this)
clean up the filter or replace (whichever your situation requires) and keep up with basic maintenance.. that is all you can do..