You might end up pushing her a way. If she wants to do it, shes only learning that she needs to be more secret with it. The evidence you had only compels her to not leave a trail.
I find it interesting that you had an addict mother and are dating this girl who wants to do coke. Think about this, its not that common yet you have found the one person who would reflect your mother.
You must be drawn to a certain character type that has heavy drug use as it's basis. If you were so anti drug why not find someone who is just as anti about them?
This is always going to be an issue and she might resent you or if you reach a snag in the relationship it might be something she does because she knows you won't like it.
I say you are playing with fire dating her. Even though I think all drugs should be legal.