Female Fighter Fins Almost Gone

[Annie11s]

I've got a large mixed tank where all fish seem pretty happy together but recently my line female fighter has been loosing her tail. I cannot see who is doing this to her but yesterday I noticed she has just one front side fin at her gill. I have removed her and placed her in a separate tank. I am unsure asto whether the fins can grow back. She is in such a poor state , such a timid fish

 

[techen]

Tank size and tank mates? A lot of fish don't get on with bettas and are typical kept by themselfs.

 

[Wildbetta]

Do you have a picture of your betta?  Most times fins will regrow with time and warm really clean water.  Bettas are not the best fish for communities(no matter the sex of the betta).  The betta is either the most aggressive fish in the community and in that case can easily kill or terrorize the other fish OR they are the ones that get picked on by the other fish.  There is rarely a good "in between" outcome.  
 
It would be good to know the size of the tank and what the fish in the tank are.  It might help to know so that we can recommend other options.