A lot of potheads with MMJ cards ALSO buy from illegal dealers because there are limits on how much they can buy via legal channels.
If they have a card and a cop catches them with some of the illegal pot there is no way for the officer to know its illegal (unless its a large quantity or he actually catches him buying from a known illegal dealer).
Point is, if your pothead neighbor has a MMJ card if you do ANYTHING to interfere with his use of his "medicine" YOU are in trouble.
Just call the police every single time you smell it ... they won't do anything but maybe you'll annoy them enough that the'll come bust the guy (but again, if he has a MMJ card then they will do absolutely nothing ... and then they'll probably tell you to stop calling it in).
No sir, I do not envy your situation.