Well, it looks like your gf's main concern is making sure she's not associated, so she needs to have a calm conversation with the flat mates father directly, sit him down and explain her part in this (well, lack of thereof). If he is doubtful she should offer to take a drug test to prove (she had nothing to hide anyway and even just offering clears her name)
She may want to save the friend also I assume. If the above interaction goes well, she'll hopefully have gained some trust off of the father. Next step would be to point out that calling the police (hopefully he hasn't already) will cause a scene. She needs to tell him that it will look badly on her and the daughter, and no one wants this.
A good idea would be to set up a meeting then with the weed rommate and the father, and get her to say she's sorry and that she will seek Councelling or something of thag nature. Hopefully if it's at all possible she can bring her parents too so it looks more proper. It's important to tell the weed girl to bring her parents and say this is because otherwise its deal with cops, so that she will comply; (it's her only way of getting out of trouble with the law) the parents will be disappointed enough (hopefully) and the father will fell like the girl is getting adequately punished (hopefully) and it will look better if she has parents present because it looks like a girl that just briefly went down a bad path, and if she says she'll seek help (talk groups, Councelling whatever) in front of her parents it won't look like she's just saying that.
However, failing this, makes sure the gf clears her own name first, the weed roommate was the stupid one to not put it away in a proper place, if she gets done it DEFINITELY should not be the GFs problem, the weed girl was the dumb one here.