OP here. We've been in treatment about 6 months. We made progress in "re-feeding" her so that her weight was restored (and her period came back after stopping for 3 months), but honestly her thought processes about herself and about her relationship with food haven't changed one iota. So she "does well" only to the extent that every meal she eats is monitored closely by an adult (even school lunch). Left to her own devices for a single meal, she will skip it in its entirety.
Sadly, to me... she seems worse. Her deceptive behavior has gotten more sophisticated (swiping food off the plate and hiding in her pockets if you turn your back for a second; throwing away half of her lunch before it could be monitored by her school counselor; etc.) So... honestly, I'm feeling defeated and like we'll never get her better. Her bio mom, on the other hand, declared her "all better" 3 months into the therapy and tried to make us all quit treatment. Thank god that some other parent reported some bizarre behavior that she witnessed (kid dumping food into garbage can right before heading into the school for the day) and so mom was forced to realize that there isn't a "magic cure" for this illness and that you can't trust a 13 year old anorexic when she says she doesn't need treatment any more.
I don't wish this on my worst enemy. I'm relieved for the advice I got here months ago to follow my gut and not wait and to get her into treatment asap. Things were way worse than even I had imagined and the only thing that brings me comfort is that at least we're facing it. And that maybe one day we'll all get to the other side of this utter hell.