Positive drug test, but why ?

Is it possible to fail a drug test (hair sample) by being in the presence of drugs? A girl claims she never consumed any type of drugs in any fashion but her boyfriend was a drug dealer and had things in the house. I am wondering could residue effect her test, possibly ingesting drugs unknowingly, airborne, or ped on through secretions through from the male to her caused her to have a positive test? I am not sure what the drug test was positive for.