This is a very advanced case of Fin rot. It will likely soon start attacking her body because she has no fin left. To be honest I feel this fish is past the point of conservative treatment. If you don't notice any improvement or she gets even worse then I would recommend purchasing medication to treat her. Fin rot is bacterial, so look for a gram negative antibiotic.
There are 16 cups in 1 gallon. If you are unsure how many gallons you are working with you can do a 100% water change (remove fish) and fill the tank using a measuring cup. That way you can count how many cups it takes to fill the tank. Make sure the water is the same temp, that you use conditioner, and acclimate the fish properly after the change.