I too would be so upset if anything happened to my cats and I do hope that it's not a deliberate act of cruelty regarding what happened to your friend's cat
Did the vet say that she had been poisoned? The only reason I ask is because cats can become very ill, very quickly and due to the cat in question being 13 (okay, not very old but not young either).
Three months ago, my 8 year old cat went from being a highly energetic, bouncy, Tigger like animal to a tired, lethargic skeletal shadow of his former self in a matter of days. Eventually I had to have him put to sleep. He wasn't poisoned according to the tests I had done, nor did he have any other illness such as feline aids or the like. I'll never know what happened but, the vet did say that sometimes cats simply get ill and die and you'll never know the reason why
As you said, the cat came in last Thursday and was ill, it could be that she had been poorly for some time but it hadn't shown itself outwardly until then.
Very sad though when an animal dies