Hmmm....in my experience, it has always been the males that start stuff, and usually, it is picking on my females. I have four males and three females.

First, I thought, too many cats in such a space? Maybe there isn't enough safe territory for each cat? Though, my house is tiny, and I have 7 cats...

Then, I thought, medical issue? Have they ruled that out?

I would suggest not so much separating them into rooms, but, making a loud, disturbing noise when the fight happens, and scolding the kitty. You know how you KNOW they are about to attack? If you can get them then with the noise, it might help.

Good luck to your cousin.