Tell us a bit more about this please.It is. We've used only Motorcraft and Wix air filters on it and the air system looks to be in good shape. His last air filter lasted less than 5k miles because of all the dirt roads and it was changed out on the most recent oil change.
What's triggering the super-short air filter changes? I realize you say it's dirty where he operates. But are you doing it because you think it needs it? OR do you have a vacuum gage on the intake?
Overly frequent air filter changes can actually be harmful because air filters will pass 90% of the total particulate in their entire lifespan in the first 10% of their lifecycle. (see this link: https://www.trailerlife.com/tech/diy/the-truth-about-engine-air-filtration )