I'm so sorry she is having problems again. Maybe she still smells urine from the last time she went on the couch and that is drawing her to it. I'm sure you've gotten this advice before but if you haven't tried it get some Nature's Miracle. It really worked for me on a chair cushion that was totally soaked by a cat we fostered for two weeks last year. I was amazed at how well it did.

Also have you tried Feliway? I've been using that for the past 7 weeks since Ripley sprayed my newly painted walls and new carpet. It has worked wonders (knock on wood) plus it seems to have calmed some of the riff between Ripley and Scout. You can buy it at most pet stores but I found it to be much cheaper to buy online from Petsmart, especially if they are offering free shipping. I'm also using a Feliway Comfort Zone Plug-In. Not cheap but it's cheaper than having my new furniture and carpet ruined.