OR, you could use this as an opportunity to talk to your child about drugs and why she shouldn't use them. And then let it go.
You will not be able to protect your child out in the real world. Isn't it better she be exposed to it while under your care and influence??? Rather than the first time while she's away from home and alone?
Edit: True Story. I stopped on my home from work to buy 12 crickets to feed my kids' lizards. TWELVE!! The kid at the store helping me reeked of pot, eyes glazed over, etc. I ask for 12 crickets. He gets a one gallon ziplock and puts in TWO SCOOPS of crickets. Easily several hundred crickets, if not more. I tell him, "Dude, that is way more than 12." He says, "Just take 'em man," I go home with the crickets, call my kids around the kitchen table, and show them the affect of pot on the brain....makes you stupid, give you bad judgment. My kids, grown now, all still remember that lesson. Don't miss your opportunity.