I don't think she's going to make it. Euthanasia is probably a good plan, before the tail rot gets any further. If it's that fast, it's going to start moving up her tail, and she's going to be in a lot of pain.
Can you get some clove oil? Maybe from a natural grocer? That's a good method. Put her in a smallish container of water in a dimly lit area to help keep her calm. Add a couple of drops of clove oil to a cup of water, shake it hard to mix them, and slowly add the clove oil mixture to her container. Keep doing that every 5-10 minutes until she stops breathing, then add a bunch more clove oil and just leave her for a while. Painless, gentle death. The worst that could happen with this method is the fish will notice the clove oil being added and get a bit scared, but in her state she'll probably pass out before she notices.
Can you get some clove oil? Maybe from a natural grocer? That's a good method. Put her in a smallish container of water in a dimly lit area to help keep her calm. Add a couple of drops of clove oil to a cup of water, shake it hard to mix them, and slowly add the clove oil mixture to her container. Keep doing that every 5-10 minutes until she stops breathing, then add a bunch more clove oil and just leave her for a while. Painless, gentle death. The worst that could happen with this method is the fish will notice the clove oil being added and get a bit scared, but in her state she'll probably pass out before she notices.