The Betta isn't going to starve any time soon. Unlike mammals and birds that use most of the food they eat to keep warm, fish take their body temperature from the surrounding water. This means any food they eat gets used for growth and movement. The fish is full grown and isn't an active swimmer. She should be able to survive on fat reserves for several months without any issues. Having said that, if she doesn't start eating within a week and can't swim properly, then it's probably time to euthanise. But give her a week to recover and see how she goes. It's a bit like a human having a serious accident. Most people don't eat straight after being involved in a car crash. The fish needs time to recover. If it still isn't eating or swimming normally in a week's time, then it has some serious internal issue.