Well there's Oust and odor neutralizers these days that I know some of my friends spray on them to go home to their parents after smoking pot and it seems to work.
I don't know, it's hard. You're going to tick her off if you tell her you read her e-mail, I mean she'll flip. I remember my mom doing that to me (I'm 21). It was more harmful to our relationship than anything else when she destroyed my sense of privacy. But I'm not saying you were wrong, because it's hard for me to say if I would do that or not. I mean, you just wanted to know if she was doing something harmful to her health and pretty much illegal. I wouldn't pay for that crap either you know. I would definitely take away the allowance, and I guess I would just tell her that I had definite and unquestionable reason that let me know she was smoking. Don't tell her exactly what. That might be hard, but just tell her you don't need to go into detail after she's done this to you. Sounds deceitful, but it's just an idea that pops into my head.
As for the nastiness of the e-mail and how she seems to enjoy it, etc. I want to say that it is pretty much normal. I'm certain I talked about my mom like that behind her back when I was a teen. I didn't really mean it and if she found it I would probably cry because I would feel awful. It's just the way they talk to their friends, honestly. It's nasty, mean, thoughtless, and unloving. But it's normal. We all do it, but in our hearts we love our parents.
How cool would it be if I looked at my friend in high school like "No I'm not gonna smoke, I love my mommy and I follow all the rules." We'd like to think they would. They wouldn't.
But it's not out of hate or because they want to hurt you. It's just cool, it makes them feel like they control their own lives. I think you know what I mean. I just don't want you to take that to heart, and I would bet a million bucks that if you approached her about it she would yell that you didn't understand. Because it's hard to understand. Just don't worry, she loves you.
Either way, good luck with her.