So update as I did take the hen to a vet. They diagnosed round worm and coccidia from a fecal sample! So did a round of invermectin (and again 10 days later) and some corid for 5 days. She perked up soon after and now seems almost back to normal, but just real skinny from lack of food. I let the flock free range with her one day after a few days of her in a mesh run next to theirs and all seemed well (she seemed to really want to get back to them), and at dusk she parked herself in their coop so I let her stay the night.
Unfortunately today I looked at her poo on the droppings board and it still doesn't seem great. Mostly normal but then a bright yellow substance that is tough/rubbery in consistency when I tried to break it a part. She also seemed more lethargic again in the afternoon after several days displaying more energy. I'm not sure if it was just discouragement or energy draining working her way back in the pecking order (it didn't seem too bad, at one point I separated one of the more aggressive birds in the adjacent mesh run for a time out). Anyway, I was a bit discouraged as I thought she was on the mend, but then today after being reintegrated into the flock she was a bit sleepy/lethargic again (still eating and drinking on her own though, so that's good). I'm leaving her in there and hoping she continues to get more energy and nutrients, hopefully her body can bounce back here. I didn't think she'd survive but she keeps on ticking. After a few days off from Corid I did start that again today just in case one round wasn't enough or I didn't do it for long enough.