I had a rescue pekin that lost the end of his bill in a raccoon attack when he was a little fluffy easter duckling.
Keep the bill clean. Keep the duck in a quiet clean place until it heals. I would bring it into the house to care for initially. Bring a quiet friend too if it seems stressed by loneliness.
Bathe the bill in salt water 4 times a day. You can either buy sachets of salt sold by pharmacies for neti pots, or make home made salt solution. If you buy salt sachets, use 2 packets per cup of clean water. If you make your own solution, dissolve 1 tablespoon salt in a cup of clean water. That is the ratio, although I recommend making a minimum of a quart at a time. You can wipe the bill with a gauze swab or a piece of old clean cloth dipped in salt water. Then pour salt water over the end of the bill or briefly dunk the bill into the salt water. But don't leave the salt water where the duck can drink it.
The bill might be unsightly when healed, but the duck will be able to feed herself if her pellets are served in a deep bowl as she will likely plow the pellets against the side. Mine like to fish for peas floating in a deep tub of water.
For now, while the bill is healing, offer liquid duck pellets soup, by dissolving pellets in warm water. Your duck will find it easier to "drink" than nibble.
You might consider putting Rooster Booster Poultry Cell vitamins and minerals in her drinking water to give her a boost while healing
