Blood tests? ifyour a doctor please answer or if you just know a lot plase answer?

Well my friend judy s now14 im still 12 well shes done weed but now for a while now but she has been smoking ciggarettes alot lately i told her this would happen but shes worried now ..what can she do from preventing it to show up in her bllood that she smokes ..i suggested she drink water but i dont kow if that helps?