Ok i think the point was to not spend money for a large grow

Also, it sounds like he lives in an apartment building so breaking into the duct work is probably not a good idea.

Honestly Bvarian, i wold go for the ozone gen. I have an ozn jr and it rocks my socks. I was pretty stinky too just going into flowering (at 5 weeks), and one of those puppies eliminates the smell easily. I also have pets and it knocks out that smell, so its used for my whole house (abot 1000 sq ft.) It does have that ozoney smell to begin with, but since I started flowering I think the particles are breaking up so fast hitting the stinkiness that I cant smell the ozone now.

Its a lil pricey, like 80 bucks, but imo you cant beat that deal with a stick. On the cheaper side, you can go with a diy carbon scrubber, but youd have to rerig your exhaust to do that. I would get rid of your input fan like Rev recommended, they tend to mess up the pressure, jst change it to an output and add some passive intakes.

Either way thats just my 2 cents.