Aww, I'm sorry to hear that. I had a male and a female betta in a ten gallon, sepateded by a divider. After cleaning it one day I didn't put it back properly and the male got onto the female's side. She beat him up pretty badly for that. I found that a product called "Stress Coat" works really well.
Here's what it looks like: http://pet.imageg.net/graphics/produ...757069t400.jpg
They sell smaller bottles if you don't want to buy that much.
Also, you can use aquarium salt. It will help the your betta heal quicker and it actually hinders bacteria that could infect your betta.
Just be patient. It takes a while for thier fins to grow back. My betta's fins haven't completely grown back yet, and its been a couple months (she really
got him bad).
Good luck. I hope she gets better soon.