Once you can rule out UTI (urinary tract infection) which are common in cats, you should definately move on to behavioral cures for her issue. One is Feliway and the other is Flower Essence.
This Feliway spray is a synthetic pheromone (almost like a body odor or hormone odor for cats) that mimics the scent a cat leaves behind when they rub their face/cheeks/whiskers on you. This is "marking" their territory to say "you are mine" or "I accept you as my family". So when your old cat goes to spray his territory again he will first smell the area and get a "whiff" of the Feliway and makes him think "hm, another cat has rubbed his face here . . . so this is where I am supposed to rub my face, not spray my urine". It has an overall calming/accepted effect on them.
Other treatment would be to give daily dose of medication or holistic type of treatment such as Bach Flower Remedies, Flower Essences, or vet prescribed anti-anxiety medication. This is a link to a website with some solutions that target the issues your cat is exhibiting especially the treatment called "Love My Litter Box:.
http://www.catfaeries.com/essences.html . These remedies are meant to be used for several months as it often takes time for stressful reactions to go away and more tolerant behavior to become more habitual. If you don't feel comfortable with these, then see if your vet can prescribe an anti-anxiety/anti-depressant for several months to get your kitty more emotionally adapted to the stresses that are causing this behavior.