Underage smoking

Hello all just looking for some advice. My fiancé has a 12 year old daughter that we believe is smoking weed. She now lives 2 hours away (with her mom) and started a new school and has been constantly in some type of trouble and it seems her mom is almost okay with this behavior because she is to busy dealing with her new relationship. We bought a few drug tests and anticipated testing her this last weekend but softened and chose not to. Now we are kicking ourselves in the ass with what we found on social media that we wish we did. And when I say we I primarily mean my Fiancé but I got his back in this. We are hesitant obviously because if it does come back positive what is the next step! She’s 12 and obviously her mom isn’t supervising her. My partner and myself wouldn’t feel comfortable sending her back. Not sure calling CPS would be the best route what if they want to hold her in a group home until a court date to grant custody. Call a family lawyer? (Sounds expensive) His BM is not your normal parent so their isn’t even reasoning with her. But obviously this is not okay! BM is really good at playing up the single mom card knowing the system she would get full custody and us less visitation. His daughter already hates coming to our house. No freedom like moms.