It is her money, they are her parents, it is her decision to live with her parents or not the same way as if you decided to move back home it would be entirely up to you.

Essentially it isn't your business, it's hers. If they really are terrible parents and she's fool enough to give them money she will learn in her own time. Many many people smoke weed and it doesn't make them bad people or bad parents, plenty of people with children manage to be terrible parents without it. It sounds as if they're in a tight money situation and need to calm down so they've turned to it for some relief during this time. If your girlfriend sees something wrong with it or them borrowing money unannounced from her she needs to confront them but it is neither your place to force her to do so or complain should she not.

Live apart, grow together. If you love one another all of it is inconsequential as eventually you will end up in a place together. I know what you want is probably in her best interests but how about you give her some time to get herself together instead of adding stress with so much haste? Things will pan out, a little patience may help.