A study in Canada, published in 2017 by the University of Calgary, showed that THC can appear in the blood/tissues of a person exposed to secondary smoke for upwards of 4 hours after exposure. They also showed in extreme cases (heavy smoking in a poorly ventilated room, total exposure for several hours) could show positive THC in the blood/tissues upwards of 24-48 hours.
Not weeks.
If you really want to read the study, it's on the Canadian Medical Association Journal Open which you can search for and find.
The bottom line? Be smart. It would appear to me, from reading the entire study, that a "positive urinalysis" in the military for THC would NOT occur from second-hand exposure as the levels reported, in nanograms, was below the level I remember from the many court-martials I sat upon as a member. Still...that being said, would you want a security clearance investigator to hear the words: "Oh...XXXX...she/he used to always hang out with the group, at school, the park, at concerts...and there was weed...oh yeah...but I don't "think" he/she used it..." It is VERY common in the high schools today (I know, I teach in HS) but there are times and places to be and to avoid.
In the end...honestly, as the outside observer looking in: let her decide. No pressure from you, as hard as that may be; but let her make the call.
It's a part of growing up and being responsible.
Steve
USAFA ALO
USAFA '83