This could be a tricky one .
If you do decide to "rat" them out and they discover it was you who did so ,are you prepared to take the consequences ?
You can never be absolutely sure they wouldn`t surmise or guess you were involved .
On the other hand who want`s druggies / dealers to be living next door to them with the problems that may bring.?
Maybe involving a willing third party from out of town do an anonymous tip to law enforcement being careful not to divulge anything to tie you or them to the tip off.