I left on a one week trip, and my sister didn't know how to do water changes, but my mom did. I told my sister to tell my mom to teach her how to do water changes. I told her to do 2 while I was gone. She did zero. I just got home and am filling up a bucket to do a water change.
There is no fuzzy white stuff on the affected spots.
Clean water and add a little salt to it and the fins should get better.
+1 I had a mini cycle in both of my betta tanks while I was gone because my husband didn't realize the filters needed to be primed after a power loss so the tanks were unfiltered. Both bettas got fin rot when the ammonia spiked. Daily water changes with a little bit of aquarium salt healed them both.
And if the water changes and salt don't do the trick(hopefully they will) I am 99% sure you can use melafix in a lower diluted amount. That's what I had to do with mine and I did a lot of research on it and he came out on top also I think you only use half the length of dosing as well