What is the tank temp?
In the case of fin rot I would do 3 -5 30% pwc in the next day or 2. The idea is to get as much of the old bad water out but gradually as to make the changes less stressful. To make the water she sits in as good as possible.
When they start feeling bad they might sit at the bottom and that is where the bad stuff accumulates so getting fin rot is not uncommon.
If there is a filter make sure it is running well enough to get some flow to help circulation. Not too too much but enough to move some.
Try getting her a Betta leaf or log to rest in. If you get a log feel inside to make sure there aren't any rough or snag like places in there. It would be placed near the top.
Or some hardy stem plants which can float at the top to give a place to rest in them. Alternatives for just laying at the bottom.
I have a lazy Betta and is often sitting around at the bottom sleeping under leaves from the dried plants used for tannins. (Indian Almond Leaves and Guava and Banana leaves) I already had to treat this fish as it was imported and very ill, so I am always worried about him.
After the water changes add in 1 tablespoon PER GALLON of Epsom Salt (plain no additives) to a cup or two of very hot water. ES is not really salt but magnesium sulfate and useful for bloating, constipation and over all better health. People use it for soaking injured/sprained muscles usually and for constipation.
Plus
1/4 teaspoon of aquarium salt per gallon / or natural, Kosher, sea salt no additives NO IODINE.
Both are pretty cheap at the Grocery store. Make sure no additives oils anticaking agent.
After every thing is dissolved 98-100% type dissolved.
Mix in cold water until it is tank temp.
ADD DECHLORINATOR when it isn't hot.
Then add in a fifth to a quarter of the mixture every 8 -12 hours, again the idea is to gradually change the water conditions.
If the fish looks like it is about to die you can add it in much more quickly every 1-2 hours) if it is like an emergency then adding it would be more beneficial than not and if the fish is on deaths doorstep you haven't much to loose. I have never had a fish even seem shocked about this treatment, emergency version or slower adding it in.
I have used this method many times and have had very good outcomes.
If feeding the inside of the circle pea and the fish isn't too interested remove the food after a couple hours. And net time try again with a squeeze of garlic clove juice on the pea. Try again. Repeat until the fish seems to eat it. Kinda like fasting but peas only diet. Hopefully after a couple bites it might help get the intestines moving.
If you are getting some medication, make sure to try this for a day and see if the Betta gets more active if it seems better, and improving you can continue that Epsom salts and aquarium salt treatment if you are happy with the improvement - for a week or 10 days without medication.
If the fish is not improving then medication may be needed.
Hoping for the best!
If there is mucky
The fish looks bloated and also recommend the old try feeding cooked peas.
