
Welcome to the forum!

Glad you joined us, just wish it were under better circumstances!
For a hen to act that way, something is definitely wrong. That does not necessarily mean she has a disease though. After a week, I think it is too late to isolate her, but that is always an option. There are so many different things it might be, it is really hard to get specific.
Is she laying? You might do a search for the signs and see if you think she is egg-bound.
Is her crop enlarged? She might have an impacted crop, but you'd almost certainly mentioned that since the crop would be so enlarged. But it is also possible the gizzard is impacted. You would not see that.
Perhaps she swallowed a nail or screw and it punched a hole in her gizzard while she was grinding it up.
One of her internal organs may have failed, like kidneys or liver.
I once had a hen acting a bit like that. She had injured her neck.
Or it might be some disease, like maybe coccidiosis. That's the first treatable one that comes ot mind, but it could be something else. I'd suggest you look for the signs of coccidiosis and see if you think that might be it.
What I would suggest, since she is off her feed, is that your friend use a medicine dropper to give her some liquid. Dehydration is a real risk. Just put a drop on the inside tip of her beak and see if she will swallow it. You don't want to drown her or put it down into her lungs, so just put it inside the tip of her beak. I use that syrup you feed hummingbirds since it has sugar to give her energy and I have it on hand, but sugar water or those vitamins would help. In addition to fighting dehydration, the sugar gives her energy. Maybe if she gets some energy, she will feel up to eating and drinking on her own.
Since you really don't know what the problem is, a round of antibiotics would be a reasonable precaution, though I hate to medicate without knowing exactly what I am medicating for. I would not give the antibiotic to the flock in general unless another chicken started showing signs of a disease. The antibiotics can mess up the probiotics in their digestive system and cause them unnecessary problems, but if you identify a disease, it is something you need to do.