Personally, if a friend of mine does pot (which a lot of them do), I don't care much because true friends will respect your wishes to not do it, if that be your choice. Like my friends, they respect me enough that if we're hanging out and their going to do that, or want to, they'll ask me if its okay (and idc), and they'll tell me if I want to do it, fine, but otherwise I can just hang out.
I used to be disgusted with it when I was older, but over time I stopped being so judgemental. I do believe some people do it in excessive amounts, but I think these people have other problems, and use this 'temporary high' to escape them.
As for a girl doing pot, or anything for that matter that you don't like.. you'll discover that a person, just like you, has their own characteristics and their not going to change once you begin to go out. For instance, if you go out with someone that acts flirtatious with everyone, their probally going to continue to be that way.
If they smoke or drink, they'll probally continue to do that..
Part of love is accepting and caring about a person as a whole, and loving them for everything they are, not what they could / should be in your eyes.
All the same, good luck.. but if you truly love this girl, make the move and tell her how you feel. If you don't get a response that you like, or she wants to remain friends, then you should move on and you'll find another girl that you love one day, and will love you in return.
Its part of maturing man.. you'll get through it.
EDIT: By the way, unless you have a base of reason not to do something, than you probally will end up doing it one day. My reason for not doing drugs is one of the people I was closest to in my life, my grandfather (God rest his soul), pleaded for me to stay from drugs and ciggerates on his death bed and that is a promise I shall keep with him.