AnotherHi, welcome to BYC!
Sorry for your trouble.
I agree that if she were being treated correctly she would be showing signs of improvement by now if she were going to. Thing is, it may be time to put her down now.. but there are so many things you don't tell us...
You say hen, how old is that? Does she lay, if so when was the last time? Have you seen poo, before she was dosed and hadn't eaten for a while? Were they regular? Ever had fecal float done to check for worms? Beside from the oatmeal that your tried to feed, what does she normally get to eat including treats and supplements? How many birds, in how much space, free range? How long have you had her?
Stopped moving... in the middle of the yard, in a lay box, on roost, standing up, laying down, hunched, eyes closed...? Was there signs of difficulty prior like stumbling or sneezing?
For future reference, oats is probably OK to try... but scrambled egg is good or maybe even some yogurt. Sometimes wetting the feed a little help. And hydration is key as they are more likely to die from dehydration than starvation. You can use an electrolyte mix (super simple homemade recipe), which in addition could have probiotics or vitamins added.. and would be my go to before tossing unverified cures that might actually do more harm than good. Please note we are all friends here (mostly), and I mean this as support. So for a bird who isn't drinking, I would administer a drop of water on top of the beak just below the nostrils.. they will usually instinctively gobble and swallow as it flows around into their mouth. For those who really need the help I will even do a drop of the liquid vitamins (I got bird ones from the pet dept.) straight to the beak like that. Oh but one of my favorite things to offer to try and help a bird, sometimes as a last resort would be an egg yolk, raw and from my own flock. Loaded with nutrients and micro nutrients that are so easy to digest it's what chicks are born with to sustain the for the first few days of life.
I have had to make these very difficult decisions. Even culled a 3 day old chick, just this week that I could tell was having failure to thrive. Sorry that you are faced with this. Even though we have processed our own meat/cockerels, it's much harder to cull a hen emotionally. But once I make that decision, I now that I made the best decision I could for the quality of life for myself and for the bird. But also by not allowing illness and disease to come live among the weak and spread to the flock, so for my other birds as well. I can't second guess myself. I know I made the best decision I could with the information I had available at the time. I wish I didn't have to, unfortunately if we keep pets or livestock, losing them is way too often part of the deal.
Here is a link on how to pack a bird for necropsy and they will take it with it's head off if that happens...
https://www.backyardchickens.com/threads/how-to-send-a-bird-for-a-necropsy-pictures.799747/
By the time they quit eating it's often much worse than we realize since they are prey animals and will hide it as long as they can.
Hope for a speedy recovery or quick (humane as possible) passing. And for you to be at peace with your choice even though it hurts.
....
