Sorry to hear that Frida has been unwell, hope she feels better soon.
In terms of their relationship, Frida smells of the vet clinic, which is why Rosa is reacting badly to her, she simply doesn't recognise her as Frida any more, and the smell has bad connotations.
Step 1: rub Rosa with a towel, or if she has some bedding that she lies on a lot, use that. Then rub Frida with it. This will transfer some of Rosa's scent to Frida.
Step 2: There's this thing called "The Vanilla Trick". For this you must use a
natural vanilla extract, not the little chemically vanilla essence bottles, I'm talking the more expensive natural extract stuff which should also be in the baking aisle of the supermarket.
Dab a little bit under each of their chins. Vanilla is safe and non-toxic for them, they also find it a pleasant scent, and it tricks them into thinking they both smell the same and pleasant to one another.
Step 3: If the above don't work - keep them separate for a few days, and try re-introducing them gradually as if you were introducing a new cat to the household.
Good luck