Just to come to everyones rescue it seems, thats normal. When you put a new male(or any aggressive fish) into a new home he will FLARE. At everything and everyone one after a couple days. The first few days are very shy betta inhabbits your aquatic dwelling but as soon as they know they are safe BLAM its all over. Excessive flaring of the gills and fins will almost "stretch them" apart. This isnt terrible, if it looks like its getting worse, 50% water change, and make sure your getting your salt put in the tank. I know its a betta, but the aquarium salt keeps a lot of fungus and other nasties away(ick included).
When I put a male and a female in a 55 gallon together, he took a GIANT nip out of her caudal fin(back fin) and it looked terrible. I thought she would never heal, but after a day or two of aloe treatments, I noticed a white rim around where she was nipped. Thats the fin regrowing in most cases. And fins do regrow but they probably will not look quite as beautiful as before.
Another instance I was breeding crowntails, the male was full of testosterone for male fishies and was struting his stuff and after about 5 days EVERY SINGLE peice of fin between the rays(point rigid part of fish fin) was completely missing. Within a week or two of aloe he was better.
This magical aloe is in most water treatments, they have two dosing instructions. One for neutralizing the chlorine and one for fin regrowth. The fin regrow is usually double dosage, but dont quote me. StartRight and melafix are the two that I have the easiest access to and what I use.
I HOPE THAT HELPED!