I noticed my filter would suck in my betta's fins, so I blocked the intake with filter foam. I used the Aquaclear
brand foam, but any foam will do. I have some foam in the intake to prevent my fish from being sucked in and at the outlet so the current isn't too strong. I noticed sooner because the new filter was the first thing Luigi wanted to investigate. Since Luigi couldn't last in the take for a full minute before getting himself stuck in the filter, I stuffed the inlet and outlet with foam right away. It only grabbed the tip of his tail, so he was able to swim away without damage. I haven't seen him get sucked in since I added the foam, so it does work if you are worried about chimchar getting sucked up again. :)
I'm not sure how long it will take his fins to grow back. It depends on the fish and how much fin was lost. I hope it grows back soon.