There's medication for fin rot. You can also use aquarium salt. Sometimes doing more frequent water changes and keeping the water pristine will allow the fish to heal.
Fin rot is opportunistic. It's a disease or fungus that can set in from a torn fin or it's caused by poor water conditions. Once the rot spreads to the body you can't save the fin and the disease goes into the body and the fish succumbs to the disease.
If your other fish look healthy that's good. Make sure your water is pristine. Bad water conditions creates a host of problems for fish.