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Hello. Thanks for the question.
You can use Fish Mox, but it's unlikely to have any antibacterial properties. Typically, we would use Clavamox or Amoxicillin or Cephalexin to treat an infection like this. I understand the difficulty with her being feral.
You may just have to let it heal on it's own. It sounds like it has already abscessed out and those are the holes you're seeing. If you can get close enough to use a topical antibiotic ointment, something like Neosporin would be fine.
If she stops using the limb, then I would want to try to use a live trap and get her to a vet. Otherwise, try the topical antibiotics +/- the fish mox and see if it helps.
I hope this helps, let me know if you have any other questions.