Why didn't my betta fight back, lost part of his tail. How do i heal it?

My ADF is a little bit stupid and thinks that anything red is food (bloodworm) my betta was all chill eating a blood worm when randomly my ADF nipped of a section of his tail. ADF not ACF so don't start with that please. I know Betta's can be aggresive but instead he is still just all chill and i don't understand why my frog is still alive since he lost a section of his tail! do i need to quarantine my betta? Will the part of the fin grow back properly? is he in pain? what should i do? the tank is 20 gallons long planted with caves so i don't think he has a space problem compared to the cup i got him in.