Total speculation on my part as well,  but - people who are completely innocent of using drugs [or of certain drugs] are not uncommonly quite unaware of what a user on a particular drug looks/acts like. There are actually quite a lot of people who have no exposure, and/or not enough to know that someone's "odd" behavior is the result of drugs.  OR  Say the mother has occasionally used a neighbor teen to baby sit, not seen the girl high because she's an opportunistic, not a regular, user.  

A truly awful pattern that I've noticed around the urban/suburb Interior is young occasional users [often a relative or neighbor's kid] that have previously been trusted by parents, but are getting sucked deeper into drug culture, taking their charges 'partying' and either losing track of the child, or actually trading the kid off as a 'party toy' in exchange for their own inclusion and high.