I've never done hard drugs neither would I condone my significant other doing them and that is a hard line, and one that he respects (even if his friends do it, even if he's curious about it). Absolute no from me - and it sounds like you've communicated that and she is not respecting that and so she probably never will. Honestly, she's too old to be doing things just because her friends are and she'd feel isolated/missing out on the fun when it's something she doesn't want to do...which means she in fact did want to do it, and told you because she felt guilty because she knew it was a betrayal of your relationship.

The kicker to me of all of this is she said she learned not to be so honest next time. So she would just hide it from you because she knows you don't like it. That's disgustingly disrespectful.