This will probably go against every "how to parent" therapist and only you know what "gets" to your kid, but you have two great opportunities here.
1. Call her on it and slightly embarrass her. That age kid is immune to scolding and preaching. There's nothing that speaks to a teenager worse than embarrassment.
2. Keep it between you and her and make sure she knows it. This is a huge opportunity to connect. I'm a big believer in being a parent first (see #1
, but you're at the age when dad is the bad guy and mom turns into a friend.
Know you aren't alone, some kids are just more sneaky than others. They're all talking/thinking about that stuff now anyway.
If it gets worse, spring back to mom role and strip everything she loves until you trust her- phone, laptop, all but 5 sets of clothes, makeup, bedroom door, etc.
I have the best kid a parent could ask for (college sophomore, turns 18 tomorrow), but I almost strangled her during her sophomore year of hs
PS- the social media rule of my house was that I had every password. If she changed it and didn't tell me, she lost it til her 18th birthday. She can change her passwords tomorrow