The problem I am willing to bet is that his problem is worn oil rings on a piston type compressor which is causing the odor, or water that has molded in the tank. In either case an inlet filter will not help and he will need an outlet filter. One other point which is frequently overlooked is that an Old air compressor tank that has rusted internally should be tested. Also if a tank does not have an ASME tag it should be replaced after 7 years. Of course no one does this until a tank ruptures and injures or kills someone and Workman’s Compensation (OSHA in the US comes down on them like a ton of bricks) ( 7 Years and ASME not needing periodic testing unless rusted and damaged are based on Quebec, Canada regulations, YMMV depending on jurisdiction)
That being said my apologies for having my eyes slide over INLET, of course you are correct and there would be no problem using any rigid container to filer the intake air.
I am a retired paramedic who cross trained in Industrial health and safety and worked in a factory employing over 1800 workers making locomotives (The old Montreal locomotive Works which sadly has been off shored), so I am very sensitive to safety concerns and trained to look for them needless to say. And am not some ‘hobbyist’ who thinks they are an expert in everything because it says so on some webpage