it is my experience also that one quality filter should be enough to remove the smell from a 4x4. like you said with negative air pressure and filtered exhaust it should be enough. so the issue is that he is recirculating and whatever filter he has isnt handling the load of all the odors. so he needs to replace it or add an additional, bigger, or higher quality filter to take some of the load off the one he is using currently.
what i do is i have my veg tent recirculating air with a filter inside it, essentially acting as a carbon scrubber for the whole room, and then i have my flower tent exhausting air out a window through a 2nd filter. the filter in the veg tent reduces the load on the exhaust of the flower tent. but if the exhaust of the flower tent is depleted/too small/low quality carbon you will still get smells.