Well, we can agree to disagree, because I still say there is no reason for that kind of aggression.
Just like in dogs, a dog that wants to rip another dog to shreds for territorial reasons is not accepted, so why should it be in cats?
Yep, again, it is up to the owner to decide. Inputting because it was asked for. I am really surprised at the amount of surprise over the attack though, and hope those who read will understand that this kind of attack can happen from mentally balanced cats.
Had my boss glance over my shoulder earlier. He has owned pretty much anything under the sun (including a rather scary cobalt tarantula, but the whole species is in a foul mood). He said:
"let me guess, chickens?"
Me: yup
Boss: What's happening in the land of chickens today
Me: ...You're pretty familiar with cats, right?
Him: *gives me a look that tells me I have just asked the stupidest question known to man*
Me: Well, let me ask you this. Adult female cat, used to living alone, has a kitten move in. Kept separate, but the kitten gets out. What would you expect the interaction to be?
Him: That kitten would get the holy @$%#%# kicked out of it.
I've seen it way too many times to be shocked with adult females bonded to owners, used to living alone, etc. Most recently, it was a relative's Oriental. Did all the same things, including *intently* waiting by the door to tear into the new young female. Not hiss at. Tear into. My own cat is a lovely and intelligent little thing, super affectionate, smart as a whip, does tricks on command, lovingly grooms our dog and rabbits...but an adult female and used to living alone. We actually consciously made a decision not to get a kitten because of the risk and higher stress for us all involved, and because we have seen heightened aggression from her in regards to kittens before. We went with the easiest type of cat to introduce to a cat like her. Adult male, slightly larger, mellow, about the same age. Took five months of itty bitty baby steps, but they will now play and occasionally lay near each other without aggression from our female. More importantly, she has just begun to allow him access to us and our attention/affection. Big step for her. She has never displayed any behavior to indicate she has a mental illness/imbalance/health issue, and has never been aggressive with a human (though we bought thick gloves when we adopted the male just in case he got out and a fight broke out, because she definitely could do damage if caught in a high-stress moment like that).
So basically, I would just personally encourage your friend to euthanize for reasons regarding improper urination (again, if the health of the cat has been checked, if the box is cleaned often) such as health issues, unsanitary living conditions, unlikely to be rehomed or increased chance of abuse if rehomed. Those will be solid, unchanging reasons, where as a seemingly hurried decision made because of a deeply disturbing incident for a person to watch may change more with time. If it was just this case of aggression, it would be possible to either manage the aggression easily (one cat household since no other or unpredictable aggression was described), or to rehome to a home that no kidding knew she should be an only cat. If euthanizing, I personally would go with reasons I knew wouldn't change once the initial shock and anger wore off. Best of luck to your friend in dealing with this stressful situation.