Sorry! My bettas are both only fish as they get aggressive to anyone else in their tanks. I don't know who might have attacked.
Fin rot can result in the same kind of damage or it can set in after damage. So, it may not be anyone attacking, though I wouldn't necessarily rule it out.
The reason I have suggested the heat is because I first tried to save Jupiter by keeping the heat on in my son's room, where Jupiter lived. The kid was cooking, but when I finally got a thermometer in the tank, I was shocked by how cold it was. Heating the room did not bring the temp anywhere close to what the heater does. That's what led me to move him into a tank, even just to float.
I wish I had other suggestions for you, but I have exhausted all of my ideas.
Good luck!