I think just tell her to be prepared, JIC. You can't stop her doing what she wants, even if you are there with her iykwim.

My 17 year old SIL tells me alot. She's pretty open with her mum, but anything she doesn't tell her, she tells me. Like when she lost her virginity at 15 to a one nighter!!!!! I was stunned, but I had to remind myself that she was not me, I couldn't control her & that at least she was telling me!
I've made it clear that she can tell me anything she wants, but if I'm ever worried about her, I'll tell her mum.
Obviously I can't tell DH any of this... lol.

Just let her know you are there if it all falls apart.