Looks like it might be bumble foot, but hard to tell because it’s a side-pic. Can't see a scab, but it would be in the center of that puffy spot on her/his foot.
It never hurts to do a epsom salt soak.
I had a big bumble-foot issue last year with my chickens. Realized we had accidentally installed roosts out of leftover treated wood.

Doh! Haven’t had an issue since replacing those roosts.
You can def take care of it at home. I brought my gals inside, soaked them in an Epsom salt bath. Cleaned the area & treated with anti-bacterial gel. Used CD (Chlorine dioxide) gel for most, which totally did the trick. Just wrapped with self-adhesive bandage afterward, so they couldn’t peck the bandage off & would keep bacteria away from the area for at least a day. Most healed that way in about a week & a half. (No bumble foot operation needed.)
The worst case I had was not getting better, so I soaked her feet in Epsom salt bath for about 25 mins, pulled the scabs off the bottom of her feet. (I did not dig. No blood.) I packed the hole left with a chunk of Bentonite Clay mixed with with warm water, then wrapped again with the self-adhesive bandage & kept her inside for about three days. It only took twice treating it with the clay, and the infection was gone. I continued to wrap her feet until that spot on her foot looked pretty well healed, where she could walk on grass, dirt & poop, and I didn’t think it would be re-infected.
I put a pic of the bandages I bought from Amazon here. They work great! Have used it for myself as well. Also used it to wrap around a roost for a chicken that was stuck inside for vent gleet the last week. It softened up the edges of the stick I used for the roost, so she could comfortably sleep all night on a smaller roost than she’s used to.
Hoping you’re able to make that duck feel like her/him self again!