I am not going to make a suggestion one way or the other but will provide the scientific insight you were looking for. Are you ready, it depends. What method is the lab using and is the person doing it paying attention. Are they running 1 or 1,000 at a time.
At the hospital lab where I worked, we did them 1 at a time on a STAT basis. If you had smoked in the last 2, maybe 2.5 months, we'd nail you every time. Heck, we can catch folks who have just handled drugs (not even taken them themselves) when everything works just right. But HPLC is more of an art than a science, so it really depends on the technologist who is working.
At the giant reference labs, these tests are being ran in huge batches often at 2 in the morning buy under trained people. It wouldn't surprise me if a 1 week would be good enough but would shoot for 3 or 4 weeks to be on the safe side. And hey, if she can go 4 weeks without a smoke, why not just quit. She should be over the really gripey stage by then anyway!!