You’re not mentioning if you’re feeding the way the old owner did. You may have drastically changed her feed and that’s a big no-no.
1, put in a call to the old owner and ask exactly how they fed her and make necessary adjustments
2, have a fecal test done to check for worms
3, start feeding her FORCO which is great to help horses digest their feed
4, I know you say her teeth need no work, but have you really had the vet check them.
5, treat her for sand
These are my first instincts in adjustments for you to make for her.
Good luck and rapid weight loss is very dangerous.