hey
i can relate to this problem, i am a guy myself but when i found out my girl was smoking green i was devistated. i was so angry that she had been hiding this from me and i seriously thought about ending things. however, this was only because i didnt know the facts! i am sure your girl is exactly the same, i have come from a background where drugs are told to be completely the wrong thing to do.
anyway, i went away and did a lot of research into the effects of cannabis and now im fine with it and have toked up myself (not because i was influenced by her but as i had a serious back injury in rugby and it releaves my pain)
we have now broken up due to her proceeding onto other drugs which i am not happy with, however just tell her the facts and tell her that it isnt harmful etc and if she still doesnt like it then she doesnt really love you.
good luck pal

p.s stick to the green! nothing else