Well, considering its been nipped at, theres really no quick fix. Adding 1 tblspoon aquarium salt per gallon will help speed up the process, but I must say, it has been bitten dead smack in the center of its tail, and even if it doesn't totally heal, it still looks unique! And as long your fishys still making her way through day to day activities and not too offended by her aggressive roommate, no harm done!
Many of my fishies either came with war wounds or developed them here (yikes) and as long as its not nipped too close to the body, they continue on as normal!
Maybe throw in a few more females to distract his focus..
Good luck!