I think it's probable that KD was the main point of contact for Nathan and that JP knew very little about what was going on. She's still quite young at 23, KD is quite a bit older than her. I can imagine she was probably aware that her boyfriend was engaged in some dodgy dealings but turned a blind eye to it usually, so this would've been no different.
I even know girls who are in this situation (I have a friend whose BF is I'm pretty sure a drug dealer) and she has a kind of don't ask don't tell policy about it. It's not a good way to be, burying your head in the sand, but this is the way a lot of these things happen. The girlfriend enjoys the money made but doesn't want anything to do with the actual crime. I'm sure she just assumed this was another of his drug deals and didn't think much of it until the arrests started being made and she put two and two together. All MOO of course.