In my opinion time can heal it ..
I have a veiltail for 7.5 months , his back fin started to be rotten about three months ago ... I didn't use any medicine because I didn't think it was very serious - and now his fin gets renewed again
I bought a halfmoon betta before one month , and I put him in a tank that I couldn't clean... I moved him into a smaller one but one that I could clean without injuring him and he got a serious fin rot - but i think that the movement helped him and it seems like his back fin is growing again ... I think that it's true ^^
I hope I'll get a bigger tank for all my three bettas for my birthday next year ... And I'll separate it of course .