Because it has only been recently that Edo has been more aggressive, it could be hormones. It could also be that Edo now knows that she can control the older kitty. She may have been unsure at first as this home was not her "territory" then. It could also be as you had suggested, the older kitty is not feeling well and Edo can smell this weakness.
I find even keeping cats separated for a time, unfortunately there is always those unpleasant memories that cats have from the first intros. To reverse that they have to replace the "unpleasant" memories to "pleasant" memories. While they cats are separated after the spay, perhaps your friend can feed the cats their favourite treats at the closed door, so they can smell each other while they are eating.
I still can't get Sweet Pea and Puddles to get along and it has been years :sad:, so you are right, it may be a long haul.
Cat maid to:
Jasper, male Ragdoll ?? (approx 10 yrs)
Rose semi feral, a cpietra rescue, female tabby (approx 7 yrs)
Sweet Pea RIP (2004?-2014)
Puddles RIP (1996-2014)
Snowball RIP (1991-2005)
In a cat's eye, all things belong to cats.-English Proverb
“While we are free to choose our actions, we are not free to choose the consequences of our actions.” Stephen R. Covey