How does the law work if say you get busted with a small home grow, would your girl friend also be busted the same if she was only living at the home a short time ?
Present and 'involved' (by association) at the time of the commission of the crime? Then guilty of the crime.
However, the authorities are quite skilled at routinely threatening low hanging fruit, and would likely use the very points you've raised re. her relative innocence, to try and convince her that they really don't want to hammer her, but that if she doesn't give them something on you, they'll have to.
Usually all parties get arrested and a detective will start interrogating the surrounding parties into giving testimonies and evidence to convict the main suspect. If no one talks usually everyone faces charges.