I have cured many injured chickens - even ones with quite large wounds.
1. Step one is to clean any obvious wound gently. I then use Calendula gel as a gentle, natural antibiotic. Sometimes I give them Arnica in
water on a
spoon also. If the bird is in shock, it
should help. Some people use something called "Rescue Remedy" for the same purpose, but I've not had
experience with it.
2. I start them in a 15" x 21" tote with a hardware cloth +wood lid over the top and hanging
feed and waterers. Actually, I usually start with just water with a little Magnesium added as it has a calming/relaxing effect. Put the tote in a warm spot. They need quiet and you don't want other birds picking on them.
3. You said the leg was injured - have you palpated gently to make sure it's not broken? If it's just soft-tissue damage, just keeping her in the tote and warm should help a lot.
4. Sometimes a frightened chicken will die a few days later just out of the shock of the attack. I'd do what I can for her - talking calmly and reassuringly to her, and then just accept what nature's choice is. Since it sounds like she's a bit of a "special" chicken, I'd think about a spot for a nice new tree or berry shrub that would benefit from your friend if it comes to that.