Sorry to hear about your duck.
We'll just start from base one here.
Could you trim the feathers around her wound, flush it with some saline ( Homemade recipes online) using either a dish soap bottle, or a large 60ml syringe. Try to dislodge any foreign matter on the wound. Then could you post another picture for us? After that, I'd either spray the Theracyn you have on hand, or slather it with a triple antibiotic ointment like Neosporin or Prep H.
I would not bandage it.
Since we're talking a bite wound here, there is going to be some bacteria on the wound from the biter's oral flora, this, of course, predisposes the bird to infection. Do you have any antibiotics hand, if so what, and how many pills do you have?
One of my concerns is the location of the wound, it's near her oil gland, in the case that she lost her oil gland she would be unable to properly repel water, and would become waterlogged very easily. So the obvious question here is, do you see her oil gland?
I would lay some towels down for traction in the bathtub for her, the bare flooring is slippery. You'll want to keep her warm, and make sure the area is stressful free.