...and I thought having a hen set the eggs would prevent that from happening....so much for that idea.
Anyway....another thing you could use is Arnica in a gel form. This is a homeopathic remedy that Laura taught me about when one of my roosters -- Junior -- pranged his leg (Junior is no longer in flock for reasons well known, but he is when I last checked happily lording over 20 acres and 14 hens along with his partner, George). You slather this stuff on the wound. It also comes in pill form that you can put in the water.
I say, put on the Arnica, clean and dress the wound as described by Rooster and Msum, and absolutely separate the chick from the hen -- and keep the chick separated until it is at least three to four months old (when it will be at least close to the physical size of an adult hen).
In fact, you might want to go as far as keeping the chick separated from even the other chicks until the wound heals. Chicks spar and peck at each other even as young as a week or 10 days. You might want to put Terramycin in the water to ward off any infections as well.
--Ron