You're kind of asking multiple things, so I'll try to give multiple answers pertaining to each part.

So as you've already mentioned, the best outcome would be for when she finally graduates and has some real freedom. I had a very similar circumstance growing up, and honestly was miserable every moment I was at home with my mom. I won't get into the details, but she is truly, absolutely insufferable. Literally one of the worst human beings I've ever encountered (to give you a basic idea, no I'm not just being a brat who had "strict" parents. She was incredibly manipulative, fake, and narcissistic to just name a few, an absolute lunatic, yet always tried to maintain an image of "the world's best mom/friend/etc" with everyone else). Realistically, I also felt a bit bound by my situation and there wasn't much I could do when I was young. And the older I got, the more I got fed up with it and the more I started to try to break free of that garbage, which obviously escalated things exponentially as time went on. So as I got to high school, I slowly started to make "quiet rebellions" where I wouldn't listen to something she said or or wouldn't do something she'd ask me (try to force me) to. Little by little. Eventually around 3rd/ 4th year of high school, I would literally try to spend as much time as I possibly can after school just doing ANYTHING (obviously not things that are detrimental to you like drugs etc) such as going to eat with friends for as long as I can, do after school activities, talk to teachers more in depth about specific things, and as soon as I'd come home, I'd go to my room and just close the door (sadly I didn't have a lock :/ ) and start with homework or whatever. After about a year of gradually building this up, I'd literally start leaving at almost any time without saying a word, and she wouldn't try to do or say anything knowing she had absolutely no power over me at that point. Again, this has to be smooth and gradual, since in reality you can go all out right away, BUT it's possible they might try to kick you out and if you don't have a job/a friend to stay with, you're screwed (although her mom might be softer than that and would just crumble under the pressure if your girl rebelled and would just let it all slide. You and her would deff be able to evaluate her mom's personality better than I can, so see if maybe that's an option). I personally literally avoided eating her food whenever I had the chance, and thankfully had a job during the summer that would last me the year when I was in high school before bills started after lol. This way I'd get to avoid full on 3 hour yelling-lectures where both of us would be screaming at each other at the top of our lungs where she'd make it seem like I am a leech and ate everything in the house if I literally would have a bowl of soup. Now looking back when I'm gladly out of that hell, it's pretty hilarious how goddamn crazy she is, I'm really still not sure whether she's just a complete sociopath and just keeps playing these crazy games and lying to everyone left and right for entertainment/etc or if she's actually so deluded she really thinks she's right...

Anyhow, so basically your girl just needs to slowly slip from her mom's grasp, at this point she'd have to do it at a faster rate than I did since she's almost out of high school, but that's the bright side-she made it this far and she has what, half a year left? She could do it. At the minimum she could pick up some after school activities just to avoid contact with her mom as much as she can. A job would also help that. A job would also help with her having some extra indepence, both in terms of just being able to do what she wants a little more, getting things she wants, and being able to negate the bs argument of "you live under my roof, and you eat my food, you do what I say". Realistically, the only response her mom would have to these rebellions would be to eventually get a dose of reality and realize that just because she's her daughter doesn't mean she owns her and can control her. A couple verbal arguments are almost guaranteed, unless her mom is really crazy and would try to hit her, at which point a quick call to police would certainly be the best course of action. Obviously make sure your girl doesn't call in front of her right as it happens, because then in the heat of the moment her mom might escalate. She's almost there, then she can go away to college and finally live her own life, at which point her mom and dad (as well as their perception/disapproval of you is irrelevant). Life is too short to be living it for someone else/by someone else's rules.