youll obviously need to seperate the sparing partners.. THe fin should grow back in a few months as long as your fish remains healthy. Keep an eye out for infections in the damaged areas.
Yeah, I tossed (gently) the offending puffer into his own little 5 gal. I am treating the wounded tank with melafix to avoid infection and I heard it is also good to use it when fin damage occurs. You say the fins will grow back?
fins are to fish what nails are to people-they'll grow back fully-the only reall poss. issue is having them grow in bent (more an environmental cause than anything else)