ill try to sugest something helpful.

before that id like to state that what your neighbor is doing is 100% within his rights. do you complain to people who smoke cigs outside too?

with that said, smoke is smoke. its bad for you, EVEN WEED SMOKE. if i was smoking on my front porch and someone lived close enough to me that they got a good wiff of it every time i smoked and they told me they didnt like it i would make a reasonable effort to make it so my smoke did not bother them.

what you should do is help this neighbor set up a carbon filtration system in one of his rooms. buy a carbon filter, a powerful enough exaust fan to pull all that smoke out, and a little stopper for the vents in that room when hes smoking. and make sure he leaves the fan on and the plugs over the vent for a good 30 minutes at least after his done smoking. the landlord should be the one paying for this vent system.

in the same vein i think he has the right to medicate/get high whatever in his own home that he pays for untill there is a law stateing otherwise (like that town in cali).
id bitch at the landlord more than anything, get that heating/venting system partitioned. this is a very common problem. if i lived above some one that smoked cigs id bitch to high heavin to my landlord.