Do you have a hospital or quarentine tank that you could put the fish into to recover? How badly has he/she been bitten? If it is fins, they can grow back over time... if it is the body of the fish, you may need to put him in a hospital tank with meds to help prevent infection.
If you don't have a hospital tank... I would suggest getting a type of soft mesh or fine netting and making a box out of it (make sure to use something sturdy so that it can't fold in on the fish and kill it. I did that for my swordtail that was getting picked on by my aggressive guppy.
Most Importantly: You will also need to find out which fish is the aggressive fish... and think about if you want to take him back to your LFS and see if they might exchange him (mine did, but call first). Since I have gotten rid of my aggressive guppy, my swordtail is perfect again... and her fins are slowly comming on.
Try not to stress out your fish too much... as this will weaken him/her further. Think about what is going to work best for you.
That is my best suggestion without knowing how bad the damage is. I hope that helps you in some way.
