The scenario that is present in your case is very common, for example; my friends smoke marijuana (pot) all the time while I'm around them. I am 15 years of age but i am very wise for my age, and i know for a fact that second hand marijuana smoke does NOT
show up on a urine test. I am in high school and they drug test us every month and nothing has showed up on any of my tests. But if something does show up have your fiance explain to his boss or superiors his case and then look online for a store that sells detox products. This is usually a drink and will erase all signs of marijuana and other drugs.
I hope my advice helped you and have a wonderful day
P.s. Set ground rules for people that stay in your house and if they do not abide by these rules then do not let them occupy your space.