ihmeg -
You can bring a defamation action against your girlfriend's mother if she carries out any of her threats regarding your father's employment, or if her comments about your drug use start to affect your reputation adversely - either at school or in your town. Right now it is hard for you to demonstrate a reputational loss of any sort, although spreading lies about someone's drug use
assumes a certain amount of reputational injury.
You can also seek to obtain a restraining order, to prohibit her from contacting you and your family. This may be counter-productive if you and your girlfriend are to remain close, and if you ever hope to gain favor with the girl's family.
And you can send this mother a "cease and desist" letter, outlining the above actions you will take if she continues to slander you or make threats. Again, this can be counter-productive, but you must balance the risk of further alienation with your right not to be slandered.
The problems with all of the above suggestions, however, as I already mentioned, is that you have an upset (crazy?
) mom, and she will certainly be more upset if you carry out any of the actions you are able legally to take. And, despite the fact that the mother is being obnoxious and a wee bit controlling, your girlfriend probably still loves her a lot. And she is also tied to her parents financially right now, while she works on a degree. So you may hurt your girlfriend and your relationship to her in the process of remedying the situation with her mom.
Is the mother being unreasonable? I certainly think so. I have never known a parent, who objects to their "adult" child's friends, coming out first. There gets to be a point in a child's life where a parent just has to accept that they have raised the kid, and step back to let the kid start making decisions for himself/herself. And, if the decisions the kid makes do turn out to be wrong, the kid will learn that and grow a little, and the parents can then be available for support. But I, personally, think it is wrong, wrong, wrong for parents to
forbid their child or children, from seeing people. When the children are basically adults, to dictate friendships is stupid. If a parent does not respect their child, and respect that they raised the child to be a responsible human being, then that is the parent's fault, not the child's.
At any rate, you don't need relationship advice, or anyone telling you that this mother has the right to control her daughter's home environment while the daughter is living at home. But you are in a difficult spot, and there is no easy answer for you - legal or otherwise.