Marijuana smoke has never been linked to causing cancer. Literally, google can marijuana cause cancer. Leave them alone, or ask them if they can smoke on the other side of the house or in their own upstairs, if they have one. Pot smokers tend to be pretty amiable if approached in a respectful way. Don't go over there being prejudiced, just politely tell them you'd prefer they avoid getting smoke up in your windows. Say you have a baby. Whatever you do, don't get them in trouble and don't be rude.