Being a landlord is a tough balancing act sometimes regarding th rights of all the tenants.  The neighbor has a right to enjoy their apartment and if the smoking is bothersome then they can complain.  But what you need to do is to check your state laws to see if smoking is inclded in nuisance laws.  If you are ceating a nuisance then you may be able to be evicted.   Maybe smoke away from the building and apartment windows.  That might help.  Good luck.