@Junibutt
Poor little chick. You did the right thing by isolating her so she may recover without bullying. I'm not sure of how to treat or even if chickens can get stitches. I did have a young chick with a gash on it's back. I applied Vetericyn spray several times a day, but since it was a broody's chick, I left it with mama. (Chick got too stressed.) The wound eventually made a nice scab & then feathered in normally. I was worried I'd have to put it down. Now I'm both surprised & happy about the outcome.
I was more hopeful earlier in the day because she was eating drinking and playing but now she has stopped eating and drinking. Will force feed her some electrolytes and yogurt/scrambled egg paste. Will see how it goes. Chicks can surprise you. The Jubilee chick whose wing got clipped in the incubator fan recovered without any issues. Apart from couple of primary feathers missing he is indistinguishable from other chicks.