Seems she is tore up from the floor up.
It could be sour crop and a bit of gleet. The sour crop definitely gives off a septic tank smell from her mouth. A messy bottom might indicate the start of gleet, which would be the downstream result of a digestive bacterial imbalance.
ACV won't cure either, but it won't hurt either. Separate her from the flock, keep her caged up, warm, and dry. You might want to purge her system w/ epsom salt or molasses mixed w/ water. Offer this as her only hydration source for at least one day. This will help purge all the crud in her tract so you can start fresh. Get some Activia yogurt (4pk) and give one container free choice each day.
As for her bottom, clean off her dirtied feathers and keep her vent area dry. You can powder puff her vent area w/ an old tube sock filled w/ talcum powder, garden-grade sulfur powder (found at hardware or big box home centers in the garden section) or combo thereof. Sulfur powder is an antifungal. Some have mentioned having success using a short (3-day) treatment of Monistat capsules (ovules), cut each into thirds and said pieces fed morning, noon, and prior to roosting until treatment is gone.
Stay away from grains or anything which could easily compact, sour, or ferment. Scrambled eggs is a safe bet, along w/ an electrolyte solution (Pedialyte or any comparable store-brand). Gatorade or Powerade is also okay, albeit you could probably dilute 50/50. I know someone will probably jump in and say the latter aren't good for chickens. FYI, the sodium and dextrose content in the latter is actually lower than vet-grade crystalloid Ringers solution.