Check again.
Colorado legalized "medical" marijuana and the threshold for getting a prescription for it is incredibly low (a simple unfounded claim of "chronic pain" will do it...for those of you not familiar with drug culture slang there's actually an unintended pun there).
The chances that your neighbor is actually legally smoking weed is pretty doggone high (dammit, another pun...sorry). So there's probably nothing you can do about it.