Societies&Zebras - I would give her a heat lamp to see if she perks up.
I'd also give her unflavored pedialyte or make up a homemade rehydration solution. (There's a recipe here if you need it:
http://wp.me/p4JWrl-F)
I wouldn't pull her, as she's already been symptomatic around the other bird. So if what she has is contagious, then it's likely the other one already has it. Pulling them would just cause more stress for everyone, so I wouldn't in this case.
I'd also offer some probiotics if it appears to be gastrointestinal.
But I don't think it's the vitamins. I have that same product. I bought it when I ran out of my usual and I didn't see any change in the droppings. At the right concentration, the yellow color is very subtle.
I don't recommend that multivitamin for use on a regular basis, though, as it only has a few nutrients and they're not at recommended concentrations. Better than nothing, but not by much.
I like this one:
http://www.amazon.com/Lafeber-410548302 ... d+vitamins
I've also used Nekton vitamins. They're not bad at all.
Anyways, if she gets worse or fails to improve, I'd reassess.
Perhaps do a round of Amoxitex (broad spectrum antibiotic)? It would be a shot in the dark, but if she's declining and you don't have the resources to go to the vet, it's better than nothing.
Have they ever been treated for protozoa or worms? That's a very common issue in pet shop birds.