Two things to cocnsider:
1. Depends on her age. Tons of girls do drugs when theyre like 16-20. Not a big deal for some, while others head down a bad path.
2. As a former drug user, one thing i can tell you is that 'occasional line' usually means it's a habbit. Every drug user claims they use less than they do. Drugs are harder on girls than they are on guys though. If she's over 21-22 and uses regularily, you will be able to see in there face.