You need to figure out what is causing the finrot. Water that is too cool is one cause. Bettas in small bowls may be more prone to finrot since the water temperature fluctuates too much without a heater. It is also harder to keep the water clean in a bowl without a filter. You would have to do water changes almost every day in a bowl, and that in itself could cause enough stress to bring on finrot.
I have had success with antibiotic treatments such as Maracyn 1 or 2 from Mardel, Fungus Eliminator from Jungle, or Kanaplex from Seachem.