First off, welcome to BYC!
You need to quarantine her so all she can eat is layer feed. If she is getting into hay, grass and other stiff vegetation that is slowing down the GI tract by the gizzard, this will make the sour crop worse.
What you want to do is add 3 tablespoons of the ACV to a gallon of water for her. Don't give her raw ACV. No oil either. Moisten her layer feed. You only want to feed her water soluble things. If you want to offer her a treat, you can give her some saltines or dry cereal. Nothing that is sweet as that will only make the yeast grow more in the crop.
Put probiotics in the feed for added good bacteria.
Keep her vomited. If she needs it 4 times a day then it is best that you do it. Try to get as much out each time as you can, although don't force her. And remember, only hold her beak down, forward, and only for about 8 seconds, no longer so she can breathe.
You can also use vaginal cream for the yeast infection. I have had really good luck with these meds. I have tried Nystatin from the vet with no luck at all.
Get some Monostat or Gyne Lomotrin and an empty syringe. Stick the end of the syringe in the tube while pulling back, squeeze the cream tube. Fill up 1/2 cc. Squirt this down her throat 3 times a day for one week and see if she responds.
See if after 3 or 4 days, doing the above things she does not improve. Sour crop can take a while to heal, especially in an older bird. Be patient and remember, do not let her free range or get into ANY feed but moistened layer feed. You have to narrow it down and you can start here. You can still let her sleep with her bunk mates and wander around a confined area. But don't let her eat anything that is going to slow the crop down.
