Honestly, this is nothing new, and it certainly won't be the first or last time your child is offered drugs. If it were me I would praise my kid for saying no, and move on. As a PP said, you need to prepare your child for the road, not the road for your child.
However, if you are truly that gung-ho about this issue, and you believe that it is something that must be rectified, I can't see how you can tell your daughter not to name names. I don't think you can have it both ways. If you all are going into the school with guns blazing and cleaning up the drug problem, when this girl gets busted she is going to know exactly who the info came from. I think if you have convictions that are that strong you need to stand up for them and teach your daughter to do the same. (FTR, I do NOT agree with this stance)
I am, honestly, quite shocked, OP, that you are that shocked. This was pretty common when I was in school, and I have a sophomore, and it's still pretty common. All things considered pot is on the mild end of the drug spectrum. Ask any kid how many kids are crushing and snorting adderall and oxycontin.